Analyte sensor with increased reference capacity

ABSTRACT

Systems and methods of use for continuous analyte measurement of a host&#39;s vascular system are provided. In some embodiments, a continuous glucose measurement system includes an electrochemical sensor incorporating a silver/silver chloride reference electrode, wherein a capacity of the reference electrode is controlled.

INCORPORATION BY REFERENCE TO RELATED APPLICATIONS

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application is a continuation of U.S. application Ser.No. 15/153,586, filed May 12, 2016, which is a divisional of U.S.application Ser. No. 13/784,523 filed Mar. 4, 2013, now U.S. Pat. No.9,351,677, which is a continuation-in-part of U.S. application Ser. No.12/829,296 filed Jul. 1, 2010, now U.S. Pat. No. 8,828,201, which claimsthe benefit of U.S. Provisional Application No. 61/222,716, filed Jul.2, 2009, U.S. Provisional Application No. 61/222,815, filed Jul. 2,2009, and U.S. Provisional Application No. 61/222,751, filed Jul. 2,2009. U.S. application Ser. No. 13/784,523 claims the benefit of U.S.Provisional Application No. 61/706,055 filed Sep. 26, 2012. Each of theaforementioned applications is incorporated by reference herein in itsentirety, and each is hereby expressly made a part of thisspecification.

FIELD OF THE INVENTION

Systems and methods of use for continuous analyte measurement of ahost's vascular system are provided. In some embodiments, a continuousglucose measurement system includes an electrochemical sensorincorporating a silver/silver chloride reference electrode, wherein acapacity of the reference electrode is controlled.

BACKGROUND OF THE INVENTION

Diabetes mellitus is a disorder in which the pancreas cannot createsufficient insulin (Type I or insulin dependent) and/or in which insulinis not effective (Type 2 or non-insulin dependent). In the diabeticstate, the victim suffers from high blood sugar, which can cause anarray of physiological derangements associated with the deterioration ofsmall blood vessels, for example, kidney failure, skin ulcers, orbleeding into the vitreous of the eye. A hypoglycemic reaction (lowblood sugar) can be induced by an inadvertent overdose of insulin, orafter a normal dose of insulin or glucose-lowering agent accompanied byextraordinary exercise or insufficient food intake.

Conventionally, a person admitted to a hospital for certain conditions(with or without diabetes) is tested for blood sugar level by a singlepoint blood glucose meter, which typically requires uncomfortable fingerpricking methods or blood draws and can produce a burden on the hospitalstaff during a patient's hospital stay. Due to the lack of convenience,blood sugar glucose levels are generally measured as little as once perday or up to once per hour. Unfortunately, such time intervals are sofar spread apart that hyperglycemic or hypoglycemic conditionsunknowingly occur, incurring dangerous side effects. It is not onlyunlikely that a single point value will not catch some hyperglycemic orhypoglycemic conditions, it is also likely that the trend (direction) ofthe blood glucose value is unknown based on conventional methods. Thisinhibits the ability to make educated insulin therapy decisions.

A variety of sensors are known that use an electrochemical cell toprovide output signals by which the presence or absence of an analyte,such as glucose, in a sample can be determined. For example, in anelectrochemical cell, an analyte (or a species derived from it) that iselectro-active generates a detectable signal at an electrode, and thissignal can be used to detect or measure the presence and/or amountwithin a biological sample. In some conventional sensors, an enzyme isprovided that reacts with the analyte to be measured, and the byproductof the reaction is qualified or quantified at the electrode. An enzymehas the advantage that it can be very specific to an analyte and also,when the analyte itself is not sufficiently electro-active, can be usedto interact with the analyte to generate another species which iselectro-active and to which the sensor can produce a desired output.Such conventional sensors can employ a silver/silver chloride referenceelectrode. Over time, such a reference electrode becomes depleted, assilver ion is converted to silver metal. As silver ion is depleted,reference electrode capacity decreases, reducing the stability of thereference electrode such that glucose sensor becomes less linear.

SUMMARY OF THE INVENTION

In a first aspect is provided an implantable electrochemical sensor formeasuring an analyte concentration in a host, comprising: a workingelectrode configured to measure a concentration of an analyte; and asilver/silver chloride reference electrode, wherein at least a portionof the silver/silver chloride reference electrode is covered with anenzyme layer, wherein the enzyme layer is configured, in vivo, togenerate hydrogen peroxide upon exposure to a substrate, whereby thehydrogen peroxide regenerates silver chloride of the reference electrodesuch that a reference capacity of the reference electrode is increased.

In an embodiment of the first aspect, the enzyme is an oxidase enzyme.

In an embodiment of the first aspect, the enzyme is glucose oxidase.

In an embodiment of the first aspect, the substrate is selected from thegroup consisting of glucose, urate, ascorbate, citrate, L-lactate,succinate, D-glucose, and ethanol.

In an embodiment of the first aspect, the analyte is glucose.

In an embodiment of the first aspect, the substrate is the analyte.

In an embodiment of the first aspect, the reference electrode comprisesa chloridized elongated silver body.

In an embodiment of the first aspect, the sensor further comprises adiffusion barrier configured to substantially block diffusion ofhydrogen peroxide between the silver/silver chloride reference electrodeand the working electrode.

In an embodiment of the first aspect, the diffusion barrier is adiscontinuity of a sensor membrane between the silver/silver chloridereference electrode and the working electrode.

In an embodiment of the first aspect, the diffusion barrier is a spatialdiffusion barrier.

In an embodiment of the first aspect, the diffusion barrier is aphysical diffusion barrier.

In an embodiment of the first aspect, the diffusion barrier is atemporal diffusion barrier.

In a second aspect is provided an electrochemical sensor for measuringan analyte concentration in a host, comprising: a working electrodeconfigured to measure a concentration of an analyte; and a referenceelectrode, wherein at least a portion of the reference electrode iscovered with an enzyme layer, wherein the enzyme layer is configured, invivo, to generate a reference electrode regenerating species uponexposure to a substrate, whereby the reference electrode regeneratingspecies regenerates a component of the reference electrode such that areference capacity of the reference electrode is increased.

In an embodiment of the second aspect, the enzyme is an oxidase enzyme.

In an embodiment of the second aspect, the enzyme is glucose oxidase.

In an embodiment of the second aspect, the substrate is selected fromthe group consisting of glucose, urate, ascorbate, citrate, L-lactate,succinate, D-glucose, and ethanol.

In an embodiment of the second aspect, the analyte is glucose.

In an embodiment of the second aspect, the substrate is the analyte

In an embodiment of the second aspect, the reference electrode comprisesa chloridized elongated silver body.

In an embodiment of the second aspect, the reference electroderegenerating species is hydrogen peroxide.

In a third aspect is provided an electrochemical sensor for measuring ananalyte concentration in a host, comprising: a working electrodeconfigured to measure a concentration of an analyte; and a referenceelectrode comprising a material that depletes during sensor use, whereinthe material is configured to be regenerated during sensor use, andwherein a rate of material depletion during sensor use substantiallycorrelates with a rate of material regeneration during sensor use over atime period.

In an embodiment of the third aspect, a correlation between the rate ofmaterial regeneration and the rate of material depletion is positive.

In an embodiment of the third aspect, the sensor is configured toincrease the rate of material regeneration responsive to an increase inthe rate of material depletion.

In an embodiment of the third aspect, the sensor is configured todecrease the rate of material regeneration responsive to a decrease inthe rate of material depletion.

In an embodiment of the third aspect, at least a portion of thereference electrode is covered with an enzyme layer.

In an embodiment of the third aspect, the enzyme is an oxidase enzyme.

In an embodiment of the third aspect, the enzyme is glucose oxidase.

In an embodiment of the third aspect, the analyte is glucose.

In an embodiment of the third aspect, the substrate is the analyte

In an embodiment of the third aspect, the material is silver chloride.

In an embodiment of the third aspect, the reference electrode is formedof a chloridized elongated silver body.

In a fourth aspect is provided a method for measuring an analyte in ahost in vivo, comprising: exposing an electrochemical sensor to a bodilyfluid of a host, wherein the electrochemical sensor comprises a workingelectrode and a silver/silver chloride reference electrode, wherein atleast a portion of the silver/silver chloride reference electrode iscovered with an enzyme layer; receiving a signal from the workingelectrode, wherein the signal is indicative a concentration of ananalyte in the bodily fluid; and generating hydrogen peroxide uponexposure of an enzyme in the enzyme layer to a substrate for the enzymepresent in the bodily fluid, wherein the hydrogen peroxide regeneratessilver chloride of the reference electrode, whereby a reference capacityof the reference electrode is increased.

In an embodiment of the fourth aspect, the method further comprisessubstantially blocking diffusion of hydrogen peroxide generated at thereference electrode to the working electrode.

In a fifth aspect is provided a method for manufacturing an implantablecontinuous analyte sensor comprising: depositing an enzyme-containingmaterial onto a conductive surface, wherein the conductive surfacecomprises at least a portion of a reference electrode, wherein theconductive surface comprises a conductive material; and drying thedeposited enzyme-containing material to form a membrane that covers theconductive surface, wherein the membrane is configured to produce aregenerating species that regenerates the conductive material duringsensor use.

In an embodiment of the fifth aspect, the enzyme-containing materialcomprises glucose oxidase.

In an embodiment of the fifth aspect, the analyte is glucose.

In an embodiment of the fifth aspect, the conductive material comprisessilver chloride.

In an embodiment of the fifth aspect, the conductive material comprisessilver and silver chloride, and wherein silver chloride is regeneratedby the regenerating species.

In an embodiment of the fifth aspect, the regenerating species ishydrogen peroxide.

In a sixth aspect is provided an implantable electrochemical sensor formeasuring a glucose concentration in a host, comprising: a workingelectrode configured to measure a glucose concentration; and a referenceelectrode comprising silver and silver chloride, wherein the referenceelectrode is configured to provide substantially stable referencepotential for up to eight days while continuously responding to a signalcurrent of about 20 nA from the working electrode.

In an embodiment of the sixth aspect, the reference electrode isconfigured to provide substantially stable reference potential for up tosixteen days while continuously responding to a signal current of about20 nA from the working electrode.

In an embodiment of the sixth aspect, at least a portion of thereference electrode is covered with an enzyme layer.

In an embodiment of the sixth aspect, the enzyme layer is configured, invivo, to generate a reference electrode regenerating species uponexposure to a substrate, whereby the reference electrode regeneratingspecies regenerates a component of the reference electrode such that areference capacity of the reference electrode is increased.

In an embodiment of the sixth aspect, the regenerating species ishydrogen peroxide.

Any of the aforementioned embodiments of an aspect may be employed inconnection with one or more other embodiments of an aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in one embodiment.

FIG. 1B is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in one embodiment.

FIG. 1C is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in another embodiment.

FIG. 1D is a cross-sectional/side-view schematic illustrating an in vivoportion of an analyte sensor, in another embodiment.

FIG. 1E is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in another embodiment.

FIG. 1F is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in another embodiment.

FIG. 1G is a side-view schematic illustrating an in vivo portion of ananalyte sensor, in another embodiment.

FIG. 2A is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in one embodiment.

FIG. 2B is a perspective-view schematic illustrating an ex vivo portionof the analyte sensor of FIG. 2A, in one embodiment.

FIG. 2C is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in one embodiment.

FIG. 3A is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in another embodiment.

FIG. 3B is a cross-sectional schematic illustrating an in vivo portionof an analyte sensor, in another embodiment.

FIG. 4A is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in one embodiment.

FIG. 4B is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in another embodiment.

FIG. 4C is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in another embodiment.

FIG. 5A is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in one embodiment.

FIG. 5B is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 5C is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 5D is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 6A is a cross-sectional schematic of the analyte sensor of FIG. 1A,taken on line 6-6, in one embodiment.

FIG. 6B is a cross-sectional schematic of the analyte sensor of FIG. 1A,taken on line 6-6, in another embodiment.

FIG. 6C is a cross-sectional schematic of the analyte sensor of FIG. 1A,taken on line 6-6, in yet another embodiment.

FIG. 7 is a perspective-view schematic illustrating an in vivo portionof an analyte sensor, in another embodiment.

FIG. 8A is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 8B is a close perspective schematic of the distal portion of thesensor embodiment illustrated in FIG. 8A.

FIG. 8C is a front view of the sensor embodiment illustrated in FIGS. 8Aand 8B.

FIG. 9A is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 9B is a close perspective schematic of the distal portion of thesensor embodiment illustrated in FIG. 9A.

FIG. 9C is a front view of the sensor embodiment illustrated in FIGS. 9Aand 9B.

FIG. 10A is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 10B is a close perspective schematic of the distal portion of thesensor embodiment illustrated in FIG. 10A.

FIG. 10C is a front view of the sensor embodiment illustrated in FIGS.10A and 10B.

FIG. 10D is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 11A is a perspective-view schematic illustrating an in vivo portionof a multi-electrode analyte sensor, in another embodiment.

FIG. 11B is a close perspective schematic of the distal portion of thesensor embodiment illustrated in FIG. 11A.

FIG. 11C is a front view of the sensor embodiment illustrated in FIGS.11A and 11B.

FIG. 12 is a front-view schematic illustrating an in vivo portion of amulti-electrode analyte sensor, in another embodiment.

FIG. 13A is a front-view schematic illustrating another embodiment amulti-electrode analyte sensor, during one stage of sensor fabrication.

FIG. 13B is a front-view schematic illustrating the embodiment shown inFIG. 13A, during another stage of sensor fabrication.

FIG. 13C is a front-view schematic illustrating the embodiment shown inFIG. 13A, during yet another stage of sensor fabrication.

FIG. 13D is a front-view schematic illustrating another embodiment amulti-electrode analyte sensor, during one stage of sensor fabrication.

FIG. 13E is a front-view schematic illustrating the embodiment shown inFIG. 13D, during another stage of sensor fabrication.

FIG. 13F is a front-view schematic illustrating the embodiment shown inFIG. 13D, during yet another stage of sensor fabrication.

FIG. 13G is a front-view schematic illustrating the embodiment shown inFIG. 13D, during yet another stage of sensor fabrication.

FIG. 14 is a perspective-view schematic illustrating a fatiguemeasurement device 1410.

FIG. 15 is a table summarizing the results of the performance of testsensors with conventional sensors, with respect to fatigue life.

FIG. 16 is a schematic illustrating metabolism of glucose by GlucoseOxidase (GOx) and one embodiment of a diffusion barrier D thatsubstantially prevents the diffusion of H₂O₂ produced on a first side ofthe sensor (e.g., from a working electrode E2 that has active GOx) to asecond side of the sensor (e.g., to a silver/silver chloride electrodeE1 that has active GOx).

FIGS. 17A-17D are plots illustrating capacities of reference electrodeswith no enzyme coverage (FIG. 17A), 0.00225 in² of enzyme coverage (FIG.17B), and 0.00633 in² of enzyme coverage (FIG. 17C). FIG. 17D provides acomparison of average reference capacities for the reference electrodesof FIGS. 17A-17C.

It should be understood that the figures shown herein are notnecessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples illustrate some exemplaryembodiments of the disclosed invention in detail. Those of skill in theart will recognize that there are numerous variations and modificationsof this invention that are encompassed by its scope. Accordingly, thedescription of a certain exemplary embodiment should not be deemed tolimit the scope of the embodiments described herein.

DEFINITIONS

In order to facilitate an understanding of the embodiments describedherein, a number of terms are defined below.

The term “analyte,” as used herein, is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a substance or chemical constituent in abiological sample (e.g., body fluids, including, blood, serum, plasma,interstitial fluid, cerebral spinal fluid, lymph fluid, ocular fluid,saliva, oral fluid, urine, excretions, or exudates. Analytes can includenaturally occurring substances (e.g., various minerals), artificialsubstances, metabolites, and/or reaction products. In some embodiments,the analyte for measurement by the sensing regions, devices, and methodsis glucose, calcium, sodium, magnesium, potassium, phosphorus, CO₂,chloride, blood urea nitrogen, creatinine, pH, a metabolic marker,oxygen, albumin, total protein, alkaline phosphatase, alanine aminotransferase, aspartate amino transferase, alanine transaminase,bilirubin, gamma-glutamyl transpeptidase, and hematocrit. However, otheranalytes are contemplated as well, including but not limited toacetaminophen, dopamine, ephedrine, terbutaline, ascorbate, uric acid,oxygen, d-amino acid oxidase, plasma amine oxidase, Xanthine oxidase,NADPH oxidase, alcohol oxidase, alcohol dehydrogenase, Pyruvatedehydrogenase, diols, Ros, NO, bilirubin, cholesterol, triglycerides,gentisic acid, ibuprophen, L-Dopa, Methyl Dopa, salicylates,tetracycline, tolazamide, tolbutamide, acarboxyprothrombin;acylcarnitine; adenine phosphoribosyl transferase; adenosine deaminase;albumin; alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle),histidine/urocanic acid, homocysteine, phenylalanine/tyrosine,tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers;arginase; benzoylecgonine (cocaine); biotinidase; biopterin; c-reactiveprotein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholicacid; chloroquine; cholesterol; cholinesterase; conjugated 1-βhydroxy-cholic acid; cortisol; creatine kinase; creatine kinase MMisoenzyme; cyclosporin A; d-penicillamine; de-ethylchloroquine;dehydroepiandrosterone sulfate; DNA (acetylator polymorphism, alcoholdehydrogenase, alpha 1-antitrypsin, cystic fibrosis, Duchenne/Beckermuscular dystrophy, glucose-6-phosphate dehydrogenase, hemoglobin A,hemoglobin S, hemoglobin C, hemoglobin D, hemoglobin E, hemoglobin F,D-Punjab, beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1,Leber hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium vivax,sexual differentiation, 21-deoxycortisol); desbutylhalofantrine;dihydropteridine reductase; diptheria/tetanus antitoxin; erythrocytearginase; erythrocyte protoporphyrin; esterase D; fattyacids/acylglycines; free β-human chorionic gonadotropin; freeerythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphatedehydrogenase; glutathione; glutathione perioxidase; glycocholic acid;glycosylated hemoglobin; halofantrine; hemoglobin variants;hexosaminidase A; human erythrocyte carbonic anhydrase I;17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase;immunoreactive trypsin; lactate; lactate dehydrogenase; lead;lipoproteins ((a), B/A-1, β); lysozyme; mefloquine; netilmicin;phenobarbitone; phenytoin; phytanic/pristanic acid; progesterone;prolactin; prolidase; purine nucleoside phosphorylase; quinine; reversetri-iodothyronine (rT3); selenium; serum pancreatic lipase; sissomicin;somatomedin C; specific antibodies (adenovirus, anti-nuclear antibody,anti-zeta antibody, arbovirus, Aujeszky's disease virus, dengue virus,Dracunculus medinensis, Echinococcus granulosus, Entamoeba histolytica,enterovirus, Giardia duodenalisa, Helicobacter pylori, hepatitis Bvirus, herpes virus, HIV-1, IgE (atopic disease), influenza virus,Leishmania donovani, leptospira, measles/mumps/rubella, Mycobacteriumleprae, Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus,parainfluenza virus, Plasmodium falciparum, poliovirus, Pseudomonasaeruginosa, respiratory syncytial virus, rickettsia (scrub typhus),Schistosoma mansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosomacruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellowfever virus); specific antigens (hepatitis B virus, HIV-1);succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine(T4); thyroxine-binding globulin; trace elements; transferrin;UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A;white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat,vitamins, and hormones naturally occurring in blood or interstitialfluids can also constitute analytes in certain embodiments. The analytecan be naturally present in the biological fluid, for example, ametabolic product, a hormone, an antigen, an antibody, and the like.Alternatively, the analyte can be introduced into the body, for example,a contrast agent for imaging, a radioisotope, a chemical agent, afluorocarbon-based synthetic blood, or a drug or pharmaceuticalcomposition, including but not limited to insulin; ethanol; cannabis(marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide,amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine(crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin,Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine);depressants (barbituates, methaqualone, tranquilizers such as Valium,Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens(phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics(heroin, codeine, morphine, opium, meperidine, Percocet, Percodan,Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogsof fentanyl, meperidine, amphetamines, methamphetamines, andphencyclidine, for example, Ecstasy); anabolic steroids; and nicotine.The metabolic products of drugs and pharmaceutical compositions are alsocontemplated analytes. Analytes such as neurochemicals and otherchemicals generated within the body can also be analyzed, such as, forexample, ascorbic acid, uric acid, dopamine, noradrenaline,3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC),homovanillic acid (HVA), 5-hydroxytryptamine (5HT), histamine, AdvancedGlycation End Products (AGEs) and 5-hydroxyindoleacetic acid (FHIAA).

The term “baseline” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a substantially constant signal derivedfrom certain electroactive compounds found in the human body that arerelatively constant (for example, baseline of the host's physiology,non-analyte related).

The term “calibration,” as used herein, is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the relationship and/orprocess of determining the relationship between the sensor data and thecorresponding reference data. In some embodiments, namely, in continuousanalyte sensors, calibration can be updated or recalibrated over time ifchanges in the relationship between the sensor data and reference dataoccur, for example, due to changes in sensitivity, baseline, transport,metabolism, and the like.

The terms “continuous,” “continuously,” and “continuous (or continual)analyte sensing” as used herein in reference to analyte sensing, arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refer without limitation to thecontinuous, continual, or intermittent (e.g., regular) monitoring ofanalyte concentration, such as, for example, performing a measurementabout every 1 to 10 minutes. It should be understood that continuousanalyte sensors generally continually measure the analyte concentrationwithout required user initiation and/or interaction for eachmeasurement, such as described with reference to continuous glucosesensors in U.S. Pat. No. 6,001,067, for example. These terms includesituations wherein data gaps can exist (e.g., when a continuous glucosesensor is temporarily not providing data).

The term “count,” as used herein, is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a unit of measurement of a digital signal.For example, a raw data stream or raw data signal measured in counts isdirectly related to a voltage (for example, converted by an A/Dconverter), which is directly related to current from the workingelectrode. In some embodiments, the terms can refer to data that hasbeen integrated or averaged over a time period (e.g., 5 minutes).

The term “crosslink,” as used herein, is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the formation of bonds (e.g.,covalent bonds, ionic bonds, hydrogen bonds, etc.) that link one polymer(or oligomer) chain to another, or to a process that increases thecohesiveness of one polymer (or oligomer) chain to another. Crosslinkscan be formed, e.g., through various reactions or processes, e.g.,chemical processes initiated by heat, pressure, catalysts, radiation,and the like.

The term “distal to,” as used herein, is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. In general, the term indicates an element is locatedrelatively far from the reference point than another element.

The term “domain” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to regions of a membrane that can be layers,uniform or non-uniform gradients (i.e., anisotropic) or provided asportions of the membrane.

The terms “electrical connection” and “electrical contact,” as usedherein, are broad terms, and are to be given their ordinary andcustomary meaning to a person of ordinary skill in the art (and are notto be limited to a special or customized meaning), and refer withoutlimitation to any connection between two electrical conductors known tothose in the art. In one embodiment, electrodes are in electricalconnection with (e.g., electrically connected to) the electroniccircuitry of a device. In another embodiment, two materials, such as butnot limited to two metals, can be in electrical contact with each other,such that an electrical current can pass from one of the two materialsto the other material.

The terms “electrochemically reactive surface” and “electroactivesurface,” as used herein, are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to a surface where an electrochemical reactiontakes place. As a non-limiting example, in an electrochemical glucosesensor, a working electrode measures hydrogen peroxide, H₂O₂, at itselectroactive surface. The hydrogen peroxide is produced by theenzyme-catalyzed reaction of the analyte detected, which reacts with theelectroactive surface to create a detectable electric current. Forexample, glucose can be detected utilizing glucose oxidase (GOX), whichproduces hydrogen peroxide as a byproduct. Hydrogen peroxide reacts withthe surface of the working electrode (e.g., the electroactive surface),producing two protons (2H⁺), two electrons (2e⁻) and one molecule ofoxygen (O₂), which produces the electronic current being detected.

The term “electrical potential” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the electrical potentialdifference between two points in a circuit which is the cause of theflow of a current.

The terms “electronics,” “sensor electronics,” and “system electronics,”as used herein, are broad terms, and are to be given their ordinary andcustomary meaning to a person of ordinary skill in the art (and are notto be limited to a special or customized meaning), and refer withoutlimitation to electronics operatively coupled to the sensor andconfigured to measure, process, receive, and/or transmit data associatedwith a sensor, and/or electronics configured to communicate with a flowcontrol device and to control and/or monitor fluid metering by a flowcontrol device.

The term “elongated conductive body,” as used herein, is a broad term,and is to be given its ordinary and customary meaning to a person ofordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to an elongated bodyformed at least in part of a conductive material and includes any numberof coatings that may be formed thereon. By way of example, an “elongatedconductive body” may mean a bare elongated conductive core (e.g., ametal wire) or an elongated conductive core coated with one, two, three,four, five, or more layers of material, each of which may or may not beconductive.

The term “ex vivo portion,” as used herein, is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a portion of a device (forexample, a sensor) adapted to remain and/or exist outside of a livingbody of a host.

The term “host,” as used herein, is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to plants or animals, for example humans.

The term “in vivo portion,” as used herein, is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a portion of a device (forexample, a sensor) adapted for insertion into and/or existence within aliving body of a host.

The term “interference domain” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to any mechanism of the membranesystem configured to reduce or eliminate any kind of interferents ornoise, such as constant and/or non-constant noise.

The terms “interferents” and “interfering species” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to effectsor species that interfere with the measurement of an analyte of interestin a sensor to produce a signal that does not accurately represent theanalyte measurement. In an exemplary electrochemical sensor, interferingspecies can include compounds with an oxidation potential that overlapswith that of the analyte to be measured.

The term “linear” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to a sensor's linear response to an analyte(e.g., glucose) concentration over a wide concentration range.

The term “multi-axis bending,” as used herein, is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a preference for bending inmore than one plane or about more than one axis.

The term “noise” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a substantially intermittent signal causedby relatively non-constant factors (for example, the presence ofintermittent noise-causing compounds that have an oxidation/reductionpotential that substantially overlaps the oxidation/reduction potentialof the analyte or co-analyte and arise due to the host's ingestion,metabolism, wound healing, and other mechanical, chemical and/orbiochemical factors, also non-analyte related).

The terms “operatively connected,” “operatively linked,” “operablyconnected,” and “operably linked” as used herein, are broad terms, andare to be given their ordinary and customary meaning to a person ofordinary skill in the art (and are not to be limited to a special orcustomized meaning), and refer without limitation to one or morecomponents linked to one or more other components. The terms can referto a mechanical connection, an electrical connection, or any connectionthat allows transmission of signals between the components. For example,one or more electrodes can be used to detect the amount of analyte in asample and to convert that information into a signal; the signal canthen be transmitted to a circuit. In such an example, the electrode is“operably linked” to the electronic circuitry. The terms include wiredand wireless connections.

The term “potentiostat,” as used herein, is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to an electronic instrument thatcontrols the electrical potential between the working and referenceelectrodes at one or more preset values.

The term “processor module,” as used herein, is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and are not to be limited to a special or customizedmeaning), and refers without limitation to a computer system, statemachine, processor, components thereof, and the like designed to performarithmetic or logic operations using logic circuitry that responds toand processes the basic instructions that drive a computer.

The term “proximal to,” as used herein, is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. In general, the term indicates an element is locatedrelatively near to the reference point than another element.

The terms “raw data,” “raw data stream,” “raw data signal,” “datasignal,” and “data stream,” as used herein, are broad terms, and are tobe given their ordinary and customary meaning to a person of ordinaryskill in the art (and are not to be limited to a special or customizedmeaning), and refer without limitation to an analog or digital signalfrom the analyte sensor directly related to the measured analyte. Forexample, the raw data stream is digital data in “counts” converted by anA/D converter from an analog signal (for example, voltage or amps)representative of an analyte concentration. The terms can include aplurality of time spaced data points from a substantially continuousanalyte sensor, each of which comprises individual measurements taken attime intervals ranging from fractions of a second up to, for example, 1,2, or 5 minutes or longer. In some embodiments, the terms can refer todata that has been integrated or averaged over a time period (e.g., 5minutes).

The term “reference electrode capacity” and “reference capacity,” asused herein, are broad terms, and are to be given their ordinary andcustomary meaning to a person of ordinary skill in the art (and are notto be limited to a special or customized meaning), and refer withoutlimitation to the actual or potential ability of the reference electrodeto maintain a certain stable reference potential in a response tocurrent signals from a working electrode. For example, with asilver/silver chloride reference electrode, the reference capacity canrefer without limitation to how long the reference electrode canmaintain sufficient silver chloride to maintain a certain stablereference potential in a response to current signals from a workingelectrode.

The term “sample,” as used herein, is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to a sample of a host body, for example,body fluids, including, blood, serum, plasma, interstitial fluid,cerebral spinal fluid, lymph fluid, ocular fluid, saliva, oral fluid,urine, excretions, or exudates.

The terms “sensing membrane,” “membrane,” and “membrane system” as usedherein are broad terms, and are to be given their ordinary and customarymeaning to a person of ordinary skill in the art (and are not to belimited to a special or customized meaning), and refers withoutlimitation to a permeable or semi-permeable membrane that can compriseone or more domains and constructed of materials of a few micronsthickness or more, which are permeable to oxygen and may or may not bepermeable to an analyte of interest. In one example, the sensingmembrane or membrane system may comprise an immobilized glucose oxidaseenzyme, which enables an electrochemical reaction to occur to measure aconcentration of glucose.

The terms “sensing region,” “sensor”, “sensor system,” and “sensingmechanism,” as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to the region or mechanism of a monitoringdevice responsible for the detection of a particular analyte, or to adevice, component, or region of a device by which an analyte can bequantified.

The term “sensitivity” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to an amount of electricalcurrent produced by a predetermined amount (unit) of the measuredanalyte. For example, in one embodiment, a sensor has a sensitivity (orslope) of from about 1 to about 100 picoAmps of current for every 1mg/dL of glucose analyte.

The term “sensor session,” as used herein, is a broad term and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a period of time a sensor isin use, such as but not limited to a period of time starting at the timethe sensor is implanted (e.g., by the host) to removal of the sensor(e.g., removal of the sensor from the host's body and/or removal of(e.g., disconnection from) system electronics).

The terms “substantial” and “substantially,” as used herein, are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to asufficient amount that provides a desired function.

As employed herein, the following abbreviations apply: Eq and Eqs(equivalents); mEq (milliequivalents); M (molar); mM (millimolar) μM(micromolar); N (Normal); mol (moles); mmol (millimoles); μmol(micromoles); nmol (nanomoles); g (grams); mg (milligrams); μg(micrograms); Kg (kilograms); L (liters); mL (milliliters); dL(deciliters); μL (microliters); cm (centimeters); mm (millimeters); μm(micrometers); nm (nanometers); h and hr (hours); min. (minutes); s andsec. (seconds); ° C. (degrees Centigrade).

Overview

The development of reliable, user-friendly in vivo analyte sensors hasbeen presented with several technical challenges relating to mechanicalproperties of the in vivo portion of the sensor. Sensors designed withan in vivo portion that has weak strength are more prone to the risk ofbreakage. Sensors designed with an in vivo portion that has greatstrength are often hard, and thus uncomfortable to the patient wearingthe sensor. What has been desired is a sensor design that has themechanical properties (e.g., a certain flexibility/stiffness as measuredby Young′ modulus and a certain high yield strength that reduces therisk of breakage of a bending sensor) that both lend comfort to the userand mechanical properties (e.g., fatigue life, strength) that providedurability and robustness to the sensor, thereby minimizing the risk ofbreakage. Described herein are sensor embodiments that overcome thesetechnical obstacles and possesses both the mechanical properties thatallow for comfort to the user and that minimizes the risk of breakage.For example, in one embodiment, the sensor comprises an elongatedconductive body that has a Young's modulus of from about 160 GPa toabout 220 GPa and a yield strength of at least 60 kPsi.

In some embodiments, the sensor is configured and arranged to monitor asingle analyte. However, in other embodiments, the sensor is configuredand arranged to monitor a plurality of analytes, such as 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, or more different analytes. In yet anotherembodiment, the sensor is configured to monitor at least one analytesubstantially continuously and to monitor at least one analyteintermittently. The analyte that is substantially, continuouslymonitored and the analyte that is intermittently monitored can be thesame analyte or a different one.

In some embodiments, the sensor is configured and arranged forimplantation in a host and for generating in vivo a signal associatedwith an analyte in a sample of the host during a sensor session. In someembodiments, the time length of the sensor session is from about lessthan 10 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, or 50minutes to about 1 hour, 2 hours, 3 hours, 4 hours, 5 hours or longer.In some embodiments, the time length of the sensor session is from about1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9hours, or 10 hours to about 11 hours, 12 hours, 13 hours, 14 hours, 15hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22hours, 23 hours, 24 hours or longer. In some embodiments, the timelength of the sensor session is from about less than 0.25 days, 0.25days, 0.5 days, 0.75 days, or 1 day to about 2 days, 3 days, 4 days, 5days, 6 days, 7 days, 8 days, 9 days, 10 days or longer.

The analyte sensor can be configured for any type of implantation, suchas transcutaneous implantation, subcutaneous implantation, orimplantation into the host's circulatory system (e.g., into a vessel,such as a vein or artery). In addition, the sensor may be configured tobe wholly implantable or extracorporeally implantable (e.g., into anextracorporeal blood circulatory device, such as a heart-bypass machineor a blood dialysis machine). U.S. Patent Publication No.2006-0020187-A1 describes an exemplary continuous analyte sensor thatcan be used for transcutaneous implantation by insertion into theabdominal tissue of a host. U.S. Patent Publication No. 2008-0119703-A1describes an exemplary embodiment of a continuous analyte sensor thatcan be used for insertion into a host's vein (e.g., via a catheter). Insome embodiments, the sensor is configured and arranged for in vitrouse.

By way of example and not of limitation, a wide variety of suitabledetection methods including, but not limited to, enzymatic, chemical,physical, electrochemical, immunochemical, optical, radiometric,calorimetric, protein binding, and microscale methods of detection, canbe employed in certain embodiments, although any other techniques canalso be used. Additional description of analyte sensor configurationsand detection methods that can be used can be found in U.S. PatentPublication No. 2007-0213611-A1, U.S. Patent Publication No.2007-0027385-A1, U.S. Patent Publication No. 2005-0143635-A1, U.S.Patent Publication No. 2007-0020641-A1, U.S. Patent Publication No.2007-002064-All, U.S. Patent Publication No. 2005-0196820-A1, U.S. Pat.No. 5,517,313, U.S. Pat. No. 5,512,246, U.S. Pat. No. 6,400,974, U.S.Pat. No. 6,711,423, U.S. Pat. No. 7,308,292, U.S. Pat. No. 7,303,875,U.S. Pat. No. 7,289,836, U.S. Pat. No. 7,289,204, U.S. Pat. No.5,156,972, U.S. Pat. No. 6,528,318, U.S. Pat. No. 5,738,992, U.S. Pat.No. 5,631,170, U.S. Pat. No. 5,114,859, U.S. Pat. No. 7,273,633, U.S.Pat. No. 7,247,443, U.S. Pat. No. 6,007,775, U.S. Pat. No. 7,074,610,U.S. Pat. No. 6,846,654, U.S. Pat. No. 7,288,368, U.S. Pat. No.7,291,496, U.S. Pat. No. 5,466,348, U.S. Pat. No. 7,062,385, U.S. Pat.No. 7,244,582, U.S. Pat. No. 7,211,439, U.S. Pat. No. 7,214,190, U.S.Pat. No. 7,171,312, U.S. Pat. No. 7,135,342, U.S. Pat. No. 7,041,209,U.S. Pat. No. 7,061,593, U.S. Pat. No. 6,854,317, U.S. Pat. No.7,315,752, and U.S. Pat. No. 7,312,040.

Although certain sensor configurations and methods of manufacture aredescribed herein, it should be understood that any of a variety of knownsensor configurations can be employed with the analyte sensor systemsand methods of manufacture described herein, such as those described inU.S. Pat. No. 5,711,861 to Ward et al., U.S. Pat. No. 6,642,015 toVachon et al., U.S. Pat. No. 6,654,625 to Say et al., U.S. Pat. No.6,565,509 to Say et al., U.S. Pat. No. 6,514,718 to Heller, U.S. Pat.No. 6,465,066 to Essenpreis et al., U.S. Pat. No. 6,214,185 toOffenbacher et al., U.S. Pat. No. 5,310,469 to Cunningham et al., andU.S. Pat. No. 5,683,562 to Shaffer et al., U.S. Pat. No. 6,579,690 toBonnecaze et al., U.S. Pat. No. 6,484,046 to Say et al., U.S. Pat. No.6,103,033 to Say et al., U.S. Pat. No. 6,512,939 to Colvin et al., U.S.Pat. No. 6,424,847 to Mastrototaro et al., U.S. Pat. No. 6,424,847 toMastrototaro et al., for example. The sensors described in theabove-identified patents documents are not inclusive of all applicableanalyte sensors. It should be understood that the disclosed embodimentsare applicable to a variety of analyte sensor configurations. It isnoted that much of the description of the embodiments, for example themembrane system described below, can be implemented not only with invivo sensors, but also with in vitro sensors, such as blood glucosemeters (SMBG).

The sensors of certain embodiments are configured and arranged forimplantation into body structures. In use, the in vivo portion of thesensor can be bent about one or more axes. This bending can occurintermittently, which can be frequently or infrequently, depending uponfactors such as the nature of the implantation site (e.g., the type(s)of surrounding tissue, the thicknesses of the tissue, etc.), the type oramount of host activity, and/or the sensor's configuration.

In one embodiment, the sensor is configured and arranged fortranscutaneous implantation. One exemplary transcutaneous implantationsite is the abdomen, which includes an abdominal wall with a pluralityof layers (e.g., skin, fascia, fat, muscles) that can move and/or slidetransversely with respect of one another (e.g., in response to movementby the host). The fascia can sometimes slide, stretch or move smalldistances across underlying fat or muscle tissue.

In some embodiments, when a sensor is transcutaneously implanted, the invivo portion passes through the skin and into an underlying tissuelayer. Depending upon the nature of the implantation site, the sensormay pass through two or more tissue layers. Consequently, voluntary orinvoluntary movements by the host can move the tissue layers, which inturn can apply force to the implanted sensor. Similarly, when the sensoris implanted into the host's circulatory system, such as into a vein orartery, and the host moves his arm, wrist and/or hand, forces may beapplied to the implanted sensor.

When certain forces (e.g., forces that cause the sensor to bend in anon-preferred bending axis) are applied to a conventional sensor, theycan cause damage to the sensor and/or the tissue surrounding the sensor.In contrast, the sensors of some of the embodiments are configured andarranged to bend and/or flex in response to forces applied thereto bysurrounding tissue and/or body movements. While not wishing to be boundby theory, it is believed that the capability of some sensor embodimentsto bend or flex, in response to application of forces by the surroundingtissue, reduces the risk of host tissue damage and sensor damage, whilestill maintaining sensor accuracy.

In certain embodiments, the sensor is configured and arranged for aunique combination of strength and flexibility that enables the sensorto be implanted for at least one, two, three or more days and to measureat least one analyte after implantation, while withstanding intermittentand/or repeated bending and/or flexing about multiple axes, such thatthe sensors. In some embodiments, the sensor is configured and arrangedto bend and/or flex at one, two, three or more points along its length(e.g., along a length corresponding to the in vivo portion implantedinto the host). Additionally or alternatively, the sensor may be capableof bending about a plurality of axes (e.g., multi-axis bending) and/orwithin a plurality of planes. As is described in greater detailelsewhere herein, components of the sensor may be treated, formed and/orcombined in a way to achieve the requisite combination of strength orflexibility that enables certain sensor embodiments to providesubstantially accurate continuous analyte data, while withstanding aharsh implantation environment for at least 1, 2, 3 or more days while,at the same time.

FIGS. 1A through 1C illustrate one aspect (e.g., the in vivo portion) ofa continuous analyte sensor 100, which includes an elongated conductivebody 102. The elongated conductive body 102 includes a core 110 (seeFIG. 1B) and a first layer 112 at least partially surrounding the core.The first layer includes a working electrode (e.g., located in window106) and a membrane 108 located over the working electrode configuredand arranged for multi-axis bending. In some embodiments, the core andfirst layer can be of a single material (e.g., platinum). In someembodiments, the elongated conductive body is a composite of at leasttwo materials, such as a composite of two conductive materials, or acomposite of at least one conductive material and at least onenon-conductive material. In some embodiments, the elongated conductivebody comprises a plurality of layers. In certain embodiments, there areat least two concentric (e.g., annular) layers, such as a core formed ofa first material and a first layer formed of a second material. However,additional layers can be included in some embodiments. In someembodiments, the layers are coaxial.

The elongated conductive body may be long and thin, yet flexible andstrong. For example, in some embodiments, the smallest dimension of theelongated conductive body is less than about 0.1 inches, 0.075 inches,0.05 inches, 0.025 inches, 0.01 inches, 0.004 inches, or 0.002 inches.While the elongated conductive body is illustrated in FIGS. 1A through1C as having a circular cross-section, in other embodiments thecross-section of the elongated conductive body can be ovoid,rectangular, triangular, polyhedral, star-shaped, C-shaped, T-shaped,X-shaped, Y-Shaped, irregular, or the like. In one embodiment, aconductive wire electrode is employed as a core. To such a cladelectrode, two additional conducting layers may be added (e.g., withintervening insulating layers provided for electrical isolation). Theconductive layers can be comprised of any suitable material. In certainembodiments, it can be desirable to employ a conductive layer comprisingconductive particles (i.e., particles of a conductive material) in apolymer or other binder.

In certain embodiments, the materials used to form the elongatedconductive body (e.g., stainless steel, titanium, tantalum, platinum,platinum-iridium, iridium, certain polymers, and/or the like) can bestrong and hard, and therefore are resistant to breakage. For example,in some embodiments, the ultimate tensile strength of the elongatedconductive body is from about 80 kPsi to about 500 kPsi. In anotherexample, in some embodiments, the Young's modulus of the elongatedconductive body is from about 160 GPa to about 220 GPa. In still anotherexample, in some embodiments, the yield strength of the elongatedconductive body is from about 60 kPsi to about 2200 MPa. Ultimatetensile strength, Young's modulus, and yield strength are discussed ingreater detail elsewhere herein. In some embodiments, the sensor's smalldiameter provides (e.g., imparts, enables) flexibility to thesematerials, and therefore to the sensor as a whole. Thus, the sensor canwithstand repeated forces applied to it by surrounding tissue. Onemeasurement of the sensor's ability to withstand the implantationenvironment is fatigue life, which is described in greater detail in thesection entitled “Multi-Axis Bending.” In some embodiments, the fatiguelife of the sensor is at least 1,000 cycles of flexing of from about 28°to about 110° at a bend radius of about 0.125-inches.

In addition to providing structural support, resiliency and flexibility,in some embodiments, the core 110 (or a component thereof) provideselectrical conduction for an electrical signal from the workingelectrode to sensor electronics (not shown), which are describedelsewhere herein. In some embodiments, the core 110 comprises aconductive material, such as stainless steel, titanium, tantalum, aconductive polymer, and/or the like. However, in other embodiments, thecore is formed from a non-conductive material, such as a non-conductivepolymer. In yet other embodiments, the core comprises a plurality oflayers of materials. For example, in one embodiment the core includes aninner core and an outer core. In a further embodiment, the inner core isformed of a first conductive material and the outer core is formed of asecond conductive material. For example, in some embodiments, the firstconductive material is stainless steel, titanium, tantalum, a conductivepolymer, an alloy, and/or the like, and the second conductive materialis conductive material selected to provide electrical conduction betweenthe core and the first layer, and/or to attach the first layer to thecore (e.g., if the first layer is formed of a material that does notattach well to the core material). In another embodiment, the core isformed of a non-conductive material (e.g., a non-conductive metal and/ora non-conductive polymer) and the first layer is a conductive material,such as stainless steel, titanium, tantalum, a conductive polymer,and/or the like. The core and the first layer can be of a single (orsame) material, e.g., platinum. One skilled in the art appreciates thatadditional configurations are possible.

Referring again to FIGS. 1A-1C, in some embodiments, the first layer 112is formed of a conductive material. The working electrode is an exposedportion of the surface of the first layer. Accordingly, the first layeris formed of a material configured to provide a suitable electroactivesurface for the working electrode, a material such as but not limited toplatinum, platinum-iridium, gold, palladium, iridium, graphite, carbon,a conductive polymer, an alloy and/or the like.

As shown in FIG. 1B-1C, a second layer 104 surrounds a least a portionof the first layer 112, thereby defining the boundaries of the workingelectrode. In some embodiments, the second layer 104 serves as aninsulator and is formed of an insulating material, such as polyimide,polyurethane, parylene, or any other known insulating materials. Forexample, in one embodiment the second layer is disposed on the firstlayer and configured such that the working electrode is exposed viawindow 106. In another embodiment, an elongated conductive body,including the core, the first layer and the second layer, is provided,and the working electrode is exposed (i.e., formed) by removing aportion of the second layer, thereby forming the window 106 throughwhich the electroactive surface of the working electrode (e.g., theexposed surface of the first layer) is exposed. In some embodiments, theworking electrode is exposed by (e.g., window 106 is formed by) removinga portion of the second and (optionally) third layers. Removal ofcoating materials from one or more layers of elongated conductive body(e.g., to expose the electroactive surface of the working electrode) canbe performed by hand, excimer lasing, chemical etching, laser ablation,grit-blasting, or the like.

In some embodiments, the sensor further comprises a third layer 114comprising a conductive material. In further embodiments, the thirdlayer may comprise a reference electrode, which may be formed of asilver-containing material that is applied onto the second layer (e.g.,an insulator). The silver-containing material may include any of avariety of materials and be in various forms, such as, Ag/AgCl-polymerpastes, paints, polymer-based conducting mixture, and/or inks that arecommercially available, for example. The third layer can be processedusing a pasting/dipping/coating step, for example, using a die-metereddip coating process. In one exemplary embodiment, an Ag/AgCl polymerpaste is applied to an elongated body by dip-coating the body (e.g.,using a meniscus coating technique) and then drawing the body through adie to meter the coating to a precise thickness. In some embodiments,multiple coating steps are used to build up the coating to apredetermined thickness. Such a drawing method can be utilized forforming one or more of the electrodes in the device depicted in FIG. 1B.

In some embodiments, the silver grain in the Ag/AgCl solution or pastecan have an average particle size associated with a maximum particledimension that is less than about 100 microns, or less than about 50microns, or less than about 30 microns, or less than about 20 microns,or less than about 10 microns, or less than about 5 microns. The silverchloride grain in the Ag/AgCl solution or paste can have an averageparticle size associated with a maximum particle dimension that is lessthan about 100 microns, or less than about 80 microns, or less thanabout 60 microns, or less than about 50 microns, or less than about 20microns, or less than about 10 microns. The silver grain and the silverchloride grain may be incorporated at a ratio of the silver chloridegrain:silver grain of from about 0.01:1 to 2:1 by weight, or from about0.1:1 to 1:1. The silver grains and the silver chloride grains are thenmixed with a carrier (e.g., a polyurethane) to form a solution or paste.In certain embodiments, the Ag/AgCl component form from about 10% toabout 65% by weight of the total Ag/AgCl solution or paste, or fromabout 20% to about 50%, or from about 23% to about 37%. In someembodiments, the Ag/AgCl solution or paste has a viscosity (underambient conditions) that is from about 1 to about 500 centipoise, orfrom about 10 to about 300 centipoise, of from about 50 to about 150centipoise.

In some embodiments, Ag/AgCl particles are mixed into a polymer, such aspolyurethane, polyimide, or the like, to form the silver-containingmaterial for the reference electrode. In some embodiments, the thirdlayer is cured, for example, by using an oven or other curing process.In some embodiments, a covering of fluid-permeable polymer withconductive particles (e.g., carbon particles) therein is applied overthe reference electrode and/or third layer. A layer of insulatingmaterial is located over a portion of the silver-containing material, insome embodiments.

In some embodiments, the elongated conductive body further comprises oneor more intermediate layers located between the core and the firstlayer. For example, in some embodiments, the intermediate layer is aninsulator, a conductor, a polymer, and/or an adhesive.

It is contemplated that the ratio between the thickness of the Ag/AgCllayer and the thickness of an insulator (e.g., polyurethane orpolyimide) layer can be controlled, so as to allow for a certain errormargin (e.g., an error margin associated with the etching process) thatwould not result in a defective sensor (e.g., due to a defect resultingfrom an etching process that cuts into a depth more than intended,thereby unintentionally exposing an electroactive surface). This ratiomay be different depending on the type of etching process used, whetherit is laser ablation, grit blasting, chemical etching, or some otheretching method. In one embodiment in which laser ablation is performedto remove a Ag/AgCl layer and a polyurethane layer, the ratio of thethickness of the Ag/AgCl layer and the thickness of the polyurethanelayer can be from about 1:5 to about 1:1, or from about 1:3 to about1:2.

In certain embodiment, the core comprises a non-conductive polymer andthe first layer comprises a conductive material. Such a sensorconfiguration can sometimes provide reduced material costs, in that itreplaces a typically expensive material with an inexpensive material.For example, in some embodiments, the core is formed of a non-conductivepolymer, such as, a nylon or polyester filament, string or cord, whichcan be coated and/or plated with a conductive material, such asplatinum, platinum-iridium, gold, palladium, iridium, graphite, carbon,a conductive polymer, and allows or combinations thereof.

As shown in FIGS. 1C and 1D, the sensor also includes a membrane 108covering at least a portion of the working electrode. Membranes arediscussed in detail in the section entitled “Membrane Configurations.”

FIGS. 3A and 3B illustrate another aspect of a continuous analyte sensor100, including an elongated body comprising a conductive core 110 and aninsulating layer 104 at least partially surrounding the conductive core,a working electrode body 112 in electrical contact with the conductivecore, and a membrane (not shown) covering the working electrode body.The elongated conductive body is configured and arranged for multi-axisbending. In some embodiments, such as the embodiment shown in FIG. 3A,the sensor includes a single insulated conductive core. However, inother embodiments, two, three, or more conductive cores are embedded ina single insulator. For example, FIG. 3B shows an elongated body havingthree conductive cores embedded in the insulator. The conductive core isformed of a conductive material suitable to electrically connect theworking electrode body to sensor electronics (not shown), and to provideflexible support to the sensor. The conductive material can include, butis not limited to, stainless steel, titanium, tantalum, a conductivepolymer, an alloy, or the like. In some embodiments, the conductive corecomprises an inner core and an outer core. In further embodiments, theinner core comprises a first conductive material, and the outer corecomprises a second conductive material. In alternative embodiments, theinner core comprises an insulating material (e.g., non-conductivematerial) and the outer core comprises a conductive material. Forexample, a non-conductive polymer, such as nylon filament can be used toform the core, and electrical conduction (e.g., between the workingelectrode body and sensor electronics) is provided by an outer coreformed of a conductive material, such as, for example, stainless steel,titanium, tantalum, a conductive polymer, an alloy or the like.

In some embodiments, the insulating layer 104 can be formed of any of avariety of insulating materials, such as polyurethane, polyimide, forexample, as described elsewhere herein. In some embodiments, such asthose shown in FIGS. 3A and 3B, a window 106 is formed in the insulator,such as by removal of a portion of the insulator using techniquesdescribed elsewhere herein. However, in other embodiments, no window isformed; rather, the working electrode body (as described below) isconfigured to penetrate through (e.g., by piercing) the insulator andmake physical contact (e.g., electrical contact) with the core. Incertain embodiments, one or more insulating layers can be formed fromheat-shrink material.

As shown in FIGS. 3A and 3B, the sensor includes a working electrodebody 112, which is formed of any of a variety of conductive materials,such as, platinum, platinum-iridium, gold, palladium, iridium, graphite,carbon, a conductive polymer, an alloy, and the like, for example. Theworking electrode body provides the electroactive surface of the workingelectrode. In some embodiments, the working electrode body includes astructure that can be attached to the elongated body, such as bycrimping, clamping, welding, adhesive, and/or the like, such as but notlimited to a C-clip, a washer, a foil, and the like. The workingelectrode body is applied to the conductive core, such that the workingelectrode body and the conductive core are electrically connected. Insome embodiments, at least a portion of the working electrode bodypenetrates (e.g., pierces, intersects) the insulating layer 104 tophysically contact (e.g., touch) the core, such that the workingelectrode body and the conductive core are electrically connected. Inother embodiments, the working electrode body is applied over window106, such that it makes electrical contact with the conductive corethrough the window. In some embodiments, one or more conductivematerials are applied between the working electrode body and the core tofacilitate conductivity therebetween. In some embodiments, an adhesive,such as a conductive adhesive, is applied between the working electrodebody and the conductive core, so as to attach the working electrode bodyto the core. In some embodiments, additional materials, such as but notlimited to polytetrafluoroethylene (such as is marketed under the tradename TEFLON®), are used to attach the working electrode body to theconductive core.

In some embodiments, instead of an elongated body having a plurality ofconductive cores embedded in an insulator, the sensor includes two ormore elongated bodies (e.g., bundled and/or twisted together) with atleast one of the elongated bodies having a working electrode bodyelectrically connected thereto. For example, FIG. 4C illustrates an invivo portion of a sensor including three elongated bodies, wherein eachelongated body includes a conductive core at least partially coated ininsulator. Two of the elongated bodies are shown to include windows,wherein working electrode bodies can be attached. In an alternativeembodiment, windows are not formed, and the working electrode bodies areC-clip structures that are crimped about the elongated bodies, whereinthe ends of the C-clips pierce the insulator and make physical (e.g.,electrical) contact with the underlying conductive cores. In yet anotherembodiment, the working electrode body is deposited, printed, and/orplated on the conductive core (e.g., through the window).

In some embodiments, the sensor includes a reference electrode, andoptionally an insulator applied to an ex vivo portion of the sensor(e.g., a portion of the reference electrode material exposed to airduring implantation), such as described herein.

FIG. 4B illustrates another embodiment of an analyte sensor, includingan elongated body comprising an insulator 104, a first conductive core110A embedded in the insulator and a second conductive core 110Bembedded in the insulator. The insulator comprises a first window 106Aconfigured and arranged to expose an electroactive portion of the firstconductive core, wherein the insulator also comprises a second window106B configured and arranged to expose an electroactive portion of thesecond conductive core. The elongated body is configured and arrangedfor multi-axis bending. A membrane 108 covers the exposed electroactiveportion of the first conductive core. In some embodiments, the sensorhas a relatively small diameter, such as described elsewhere herein. Forexample, in some embodiments, the smallest dimension of the elongatedbody is less than or equal to about 0.002 inches, 0.004 inches, 0.01inches, 0.05 inches, 0.075 inches, 0.1 inches, 0.25 inches, 0.5 inches,or 0.75 inches. The elongated body can be formed using a variety ofinsulators known in the art. In some embodiments, the insulatorcomprises at least one of polyurethane or polyimide.

In some embodiments, the electroactive portion of the first conductivecore is a working electrode. Accordingly, in some embodiments, the firstconductive core may be formed of platinum, platinum-iridium, gold,palladium, iridium, graphite, carbon, a conductive polymer, orcombinations or alloys thereof. In alternative embodiments, the firstconductive core comprises a core and a first layer, wherein an exposedelectroactive surface of the first layer provides the working electrode.For example, in some embodiments, the core comprises stainless steel,titanium, tantalum and/or a polymer, and the first layer comprisesplatinum, platinum-iridium, gold, palladium, iridium, graphite, carbon,a conductive polymer and/or an alloy.

In some embodiments, the second conductive core provides a referenceelectrode, and thus may be formed of a silver-containing material, suchas, a silver wire or a silver-containing polymer, for example. In someembodiments, the silver is chloridized prior to being embedded in theinsulator. In other embodiments, the silver is non-chloridized prior tobeing embedded in the insulator, and is chloridized after the silver isexposed (e.g., via window 106B).

In some embodiments, a third conductive core is embedded in theinsulator. For example, in some embodiments, the sensor is adual-electrode sensor and the third conductive core is configured as asecond working electrode. In alternative embodiments, the thirdconductive core is configured as a counter electrode, in someembodiments. The third conductive core is a wire-shaped structure formedof at least one of platinum, platinum-iridium, gold, palladium, iridium,graphite, carbon, a conductive polymer and an alloy, in someembodiments. Alternatively, the third conductive core can include a coreand a first layer, such as described herein. In some furtherembodiments, the core comprises an inner core and an outer core, such asdescribed herein.

In some further embodiments, the sensor includes two or more additionalworking electrodes. For example, the sensor can be configured to detecttwo or more analytes. Alternatively, some of the additional workingelectrodes can be configured as redundant sensors.

Referring again to FIG. 4B, in one exemplary embodiment, the sensor is aglucose sensor configured and arranged for multi-axis bending, andcomprises an elongated body comprising an insulator 104 formed of aninsulating material such as a polyurethane or a polyimide. A platinum ora platinum-iridium wire and a silver wire 110B is embedded in theinsulator. The electroactive surface (e.g., the working electrode) ofthe platinum or a platinum-iridium wire is exposed via window 106A. Thereference electrode (e.g., of the silver wire) is exposed via window106B. A membrane 108 covers at least the working electrode. In someembodiments, the membrane may cover a larger portion of the sensor. Forexample, in FIG. 4B, the membrane covers the illustrated in vivo portionof the sensor.

The sensor can be manufactured using a variety of techniques and methodsknown in the art. For example, in one embodiment, the elongated body isprovided with the insulator 104 and with the first 110A and second 110Bconductive cores embedded therein). Portions of the insulator (e.g.,windows 106A and 1060B) are then removed to expose the working electrode(e.g., located on the first conductive core, the Pt or Pt/Ir wire) andthe reference electrode (e.g., located on the second conductive core,the Ag wire). In some embodiments, the step of removing a portion of theinsulator to expose the working electrode and/or the reference electrodecomprises ablating (e.g., laser or UV ablation) a portion of theinsulator. Alternatively, portions of insulator can be removed manuallyor using methods known in the art such as grit blasting. If the silverwire is not provided as a chloridized wire, it can be chloridized priorto application of a membrane to the sensor. In some circumstances, theelectroactive surfaces are cleaned, using standard methods known in theart, prior to membrane application. The membrane 108 is then applied toat least a portion of the sensor, such as but not limited to the workingelectrode. For example, at least the portion of the membrane coveringthe working electrode includes an enzyme (e.g., glucose oxidase)selected to detect the analyte.

In some embodiments, the membrane is formed of a polymer having a Shorehardness of from about 70 A to about 55 D. In some embodiments, one ormore of the membrane domains are formed of polymers within this hardnessrange. However, in some embodiments, only the resistance domain isformed from a polymer having a Shore hardness of from about 70 A toabout 55 D. While not wishing to be bound by theory, it is believed thatsome polymers having a Shore hardness in this range are sufficientlyelastic yet resilient to withstand multi-axis bending withoutsubstantial disruption of the membrane's function. In some embodiments,the manufactured sensor has a fatigue life of at least about 1,000cycles of flexing of from about 28° to about 110° at a bend radius ofabout 0.125 inches. However, the sensors of other embodiments can havelonger fatigue lives (e.g., fatigue lives of about at least 10,000,20,000, 30,000, 40,000 or 50,000 cycles or more of flexing).

FIG. 7 illustrates yet another continuous analyte sensor of anembodiment. In this particular embodiment, the sensor 700 comprises anelongated body 702 configured and arranged for multi-axis bending. Theelongated conductive body 702 comprises a nonconductive material, aworking electrode 704 located on the elongated body, a referenceelectrode 706 located on the elongated body, and a membrane 108 coveringthe working electrode. Conductive pathways 708 and 710 connect theworking electrode and the reference electrode (respectively) to thesensor electronics (not shown). In further embodiments, the sensor isconfigured and arranged such that the fatigue life of the sensor is atleast 1,000 cycles of flexing of from about 28° to about 110° at a bendradius of about 0.125 inches. Some embodiments are configured to providelonger fatigue lives, as described in the section entitled “Multi-AxisBending.”

In some embodiments, the elongated body can be formed out of anynonconductive material that can be formed into a thin, elongatedstructure. In further embodiments, the nonconductive material is apolymer. The polymer may be a nylon or polyester filament, string orcord, etc. In some embodiments, the elongated body is non-planar, suchas described herein, and thus has a non-rectangular cross-section.However, in certain embodiments, the elongated body is planar. In someembodiments, the smallest dimension (e.g., the diameter or width) of theelongated body is less than about 0.004 inches. However, in certainembodiments, relatively larger or smaller sensor diameters areacceptable, such as described elsewhere herein.

The sensor 700 illustrated in FIG. 7 can be manufactured using a varietyof techniques known in the art. In one embodiment, a method of making aflexible continuous analyte sensor adapted for in vivo use includes thesteps of providing a non-planar elongated body 702 comprising anonconductive material, applying a working electrode 704, the referenceelectrode 706 and conductive pathways 708, 710 on the elongated body,and covering (at least) the working electrode with a membrane 108,thereby producing a sensor capable of multi-axis bending. In someembodiments, the working electrode is applied by depositing a conductivematerial (e.g., at least one of platinum, platinum-iridium, gold,palladium, iridium, graphite, carbon, a conductive polymer and an alloy)on the elongated body. In some embodiments, the conductive material isan ink, paint or paste, and is deposited using thick film and/or thinfilm deposition techniques known in the art, such as but not limited toscreen printing, jet printing, block printing, and the like. However, insome embodiments the working electrode material is plated on theelongated body, using plating techniques known in the art, such as butnot limited to electroplating. The reference electrode can be applied bydepositing a silver-containing material on the elongated body. Similarto the working electrode, the silver-containing material of thereference electrode can be deposited using thick film and/or thin filmdeposition techniques, various printing techniques known in the art,and/or plating. The membrane is applied to at least the workingelectrode using standard techniques. In particular, in some embodiments,the membrane is applied by applying a polymer having a Shore hardness offrom about 70 A to about 55 D. As described with reference to FIGS.6A-C, the membrane can include a plurality of layers and/or domains. Theoutermost domain in certain embodiments is the resistance domain, whichis configured to modulate the amount of analyte and/or other substancesdiffusing into and/or through the membrane. In some embodiments, thestep of applying a membrane comprises forming a resistance domain from apolymer having a Shore hardness of from about 70 A to about 55 D. Forexample, additional membrane domains (e.g., enzyme, interference,electrode domains, etc.) can be formed of other polymers, as is known inthe art and described with reference to FIGS. 6A-C. While the sensor canbe manufactured by hand, in some embodiments, at least one step issemi-automated. In certain embodiments, at least one step isfully-automated. In some circumstances, two or more steps aresemi-automated or fully-automated.

Various sensor configurations that can be useful in connection withcertain embodiments are described in U.S. Pat. No. 7,529,574. Forexample, the analyte sensor can have an active sensing region thatincludes an electrochemically active surface and a membrane system thatadheres to or is otherwise situated atop the electrochemically activesurface, wherein one or more protruding structures of dielectricmaterial may extend outwardly from the electrochemically active surface(or other surface below the membrane system) and serve as supportivestructure(s) to the membrane system. This particular configuration canbe desirable when forming the membrane by dip coating a liquid (e.g., aviscous liquid, or curable liquid) onto the electrochemically activesurface or other underlying surface of the sensor. The protrudingstructures are configured in a way such that they can support the liquidbefore, during, or after the curing process. The protruding structurescan be in a form of one or more rings having, e.g., sharp corners orsmooth edges, a square or semicircular cross section, or any otherdesired configuration. For example, the sensor can comprise a platinumwire coated with an insulating material (e.g., a polyimide layer), and asilver wire can be wrapped around a portion of this structure. Aretractor (e.g., of stainless steel or other suitable material) can beincorporated into the sensor. Portions of the insulating material can beremoved, e.g., by laser ablating, with the remaining insulating materialforming the protruding structures (e.g., in the form of protrudingrings). Multiple protruding structures can be spaced longitudinallyalong the surface of the platinum wire. After the laser ablationoperation, the platinum wire with protruding structures is dip coatedwith one or more membrane layers, e.g., one or more layers such as aninterference layer, a resistance layer, an enzyme layer, and the like,as discussed elsewhere herein. The protruding structures enableadditional coating material to adhere to the wire, e.g., throughcapillary action, which can be desirable in forming thicker membranes infewer steps, or by reducing the number of dip coating/curing stepsnecessary to form a particular thickness of layer.

U.S. Patent Publication No. 2007-0173711-A1 describes thin filmfabricating techniques that can be employed in the manufacture ofsensors of certain embodiments. In such techniques, a base layer orsubstrate (conducting or nonconducting) is subjected to one or moredeposition steps (e.g., metallization steps to form one or moreconductive layers and/or electrode layers, or steps wherein anelectrically insulating layer such as a polyurethane or polyimide isapplied) to form at least a portion of the sensor. For example, a baselayer that is an electrically insulating layer such as a polyimidesubstrate can be employed (e.g., self-supporting or further supported byanother material). The base layer can be a polyimide tape, dispensedfrom a reel, to facilitate clean, high density mass production, and/orproduction of sensors on both sides of the tape.

Metallization steps involve application of a conductive layer onto aninsulating layer (or other layer). The conductive layer can be providedas a plurality of thin film conductive layers, e.g., a chrome-basedlayer for chemical adhesion to the base layer, followed by subsequentformation of a thin film gold- or platinum-based layer, or achrome-based top layers on top of the thin film gold- or platinum-basedlayer. The conductive layer may also be formed of gold and/or chrome indifferent ratios and/or other adhesive/conductive layers, such astitanium, platinum, tungsten, or the like. In alternative embodiments,other electrode layer conformations or materials can be used. Theconductive layer can be applied using electrode deposition, surfacesputtering, or another suitable process step. The electrical circuit ofeach conductive layer typically comprises one or more conductive pathswith regions at a proximal end that form contacts and regions at adistal end that form sensor electrodes. Generally, etching is performedto define the electrical circuit of each layer. Alternatively, “liftoff” may be used, in which the photoresist defines a pattern prior tometal sputtering, after which the photoresist is dissolved away (alongwith the unwanted metal), and the metal pattern is left behind. Infurther embodiments, photoresisting is performed to protect themetalized pathway and electrode and photoimaging is performed to curespecified areas. For example, the conductive layer is covered with aselected photoresist coating, followed by an etch step resulting in oneor more conductive paths. An electrically insulating cover layer (ordielectric layer), such as a polymer coating, is then applied over atleast portions of the conductive layer. Suitable polymer coatings foruse as the insulating cover layer include, for example, non-toxicbiocompatible polymers such as polyimide, biocompatible solder masks,epoxy acrylate copolymers, and the like. Further, these coatings can bephotoimageable to facilitate photolithographic formation of aperturesthrough to the conductive layer to expose the electrode. Multiplemetallization steps can be employed to fabricate additional electrodes(e.g., sequentially) when intervening insulating layers are employed.For example, the resulting electrodes can be in a staggeredconfiguration, so that at least a portion of each electrode may beexposed, or the conductive layers can be directly above each other.Alternatively, multiple electrodes can be fabricated at the same time(e.g., simultaneously) on the same insulating substrate. The conductivelayers can be horizontally displaced from each other. The electrodes canbe further be configured in any way that allows the electrodes tocontact fluid when inserted into a body of a patient.

Sensors of embodiments can include conductive layers alternating withthe insulating layers. In between every two conductive layers there maybe an insulating layer that serves to isolate each conductive layer sothat there is no trace communication between the layers. Apertures canbe formed in a top insulating cover layer, or the top layer can be aconductive layer. Electrodes can be in a vertical orientation atop eachother, or spaced sideways so that they are not directly on top of eachother (e.g., horizontally displaced). Conductive pathways that lead toconductive contacts can be similarly positioned. The apertures can bemade through photolithographic development, laser ablation, chemicalmilling, etching, or the like. The exposed electrodes and/or contactscan also undergo secondary processing through the apertures, such asadditional plating processing, to prepare the surfaces, and/orstrengthen the conductive regions.

Typically, the conductive layers (or electrodes) are formed by any of avariety of methods known in the art such as photoresist, etching andrinsing to define the geometry of the active electrodes. The electrodescan then be made electrochemically active, for example byelectrodeposition of platinum black for the working and counterelectrode, and silver followed by silver chloride on the referenceelectrode. The sensor chemistry layer is then disposed on the conductivelayer by a method other than electrochemical deposition, usuallyfollowed by vapor crosslinking, for example with a dialdehyde, such asglutaraldehyde, or a carbodiimide.

One or more sensors can be formed on a rigid flat substrate, such as apolymer, glass, ceramic, composite, or metal. When finished, the sensorsmay be removed from the rigid flat substrate by a suitable method, suchas laser cutting. Other materials that can be used for the substrateinclude, but are not limited to, stainless steel, aluminum, and plasticmaterials. Flexible sensors can be formed in a manner which iscompatible with photolithographic mask and etch techniques, but wherethe sensors are not physically adhered or attached directly to thesubstrate. Each sensor thus comprises a plurality of thin filmelectrodes formed between an underlying insulating base layer and aninsulating cover layer.

A flexible electrochemical sensor can be constructed according to thinfilm mask techniques to include elongated thin film conductors embeddedor encased between layers of a selected insulating material such aspolyimide film or sheet. The sensor electrodes at a tip end of thesensor distal segment are exposed through one of the insulating layersfor direct contact with patient fluids, such as blood and/orinterstitial fluids, when the sensor is transcutaneously,subcutaneously, or intravenously placed. The proximal segment and thecontacts thereon are adapted for electrical connection to a suitablemonitor for monitoring patient condition in response to signals derivedfrom the sensor electrodes. The sensor electronics may be separated fromthe sensor by wire or be attached directly on the sensor. For example,the sensor may be housed in a sensor device including a housing thatcontains all of the sensor electronics, including any transmitternecessary to transmit data to a monitor or other device. The sensordevice alternatively may include two portions, one portion housing thesensor and the other portion housing the sensor electronics. The sensorelectronics portion could attach to the sensor portion in a side-to-sideor top-to-bottom configuration, or any other configuration that wouldconnect the two portions together.

If the sensor electronics are in a housing separated by a wire from thesensor, the sensor electronics housing may be adapted to be placed ontothe user's skin or placed on the user's clothing in a convenient manner.The connection to the monitor may be wired or wireless. In a wiredconnection, the sensor electronics may essentially be included in themonitor instead of in a housing with the sensor. Alternatively, sensorelectronics may be included with the sensor as described above. A wirecould connect the sensor electronics to the monitor. Examples ofwireless connection include, but are not limited to, radio frequency,infrared, WiFi, ZigBee and Bluetooth. Additional wireless connectionsfurther include single frequency communication, spread spectrumcommunication, adaptive frequency selection and frequency hoppingcommunication. In further embodiments, some of the electronics may behoused on the sensor and other portions may be in a detachable device.For example, the electronics that process and digitize the sensor signalmay be with the sensor, while data storage, telemetry electronics, andany transmission antenna may be housed separately. Other distributionsof electronics are also possible, and it is further possible to haveduplicates of electronics in each portion. Additionally, a battery maybe in one or both portions. In further embodiments, the sensorelectronics may include a minimal antenna to allow transmission ofsensor data over a short distance to a separately located transmitter,which would transmit the data over greater distances. For example, theantenna could have a range of up to 6 inches, while the transmittersends the information to the display, which could be over 10 feet away.The overall sensor height of sensors fabricated by such methods (frombase to top insulating layer) can be on the order of microns (e.g., lessthan 250 microns, less than 100 microns, less than 50 microns, or lessthan 25 microns). The base layer can be about 12 microns and eachinsulating layer can be about 5 microns. The conductive/electrode layerscan be several thousand angstroms in thickness. Any of these layerscould be thicker if desired. The overall width of the sensor can be assmall as about 250 microns or less or 150 microns or less. The length ofthe sensor can be selected depending upon the depth and/or method ofinsertion. For example, for transcutaneous or subcutaneous sensing, thesensor length may be about 2 mm to 5 mm, or for intravenous sensing upto about 3 cm or more.

In certain embodiments, the sensor (e.g., sensor 100) is configured andarranged for multi-axis bending. The term “bending,” as used herein, isa broad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refers without limitation tomovement that causes the formation of a curve, or a condition that ischaracterized as being not rigid or not straight. In general, astructure capable of multi-axis bending is configured for substantialbending in (e.g., within, along) two or more planes (e.g., about two ormore axes). In one exemplary embodiment, with respect to the in vivoportion of a continuous analyte sensor, there is no preferred bendingpoint or location for a bend and/or flex to occur. Accordingly, in someembodiments, the sensor is configured and arranged to bend along aplurality of planes, such as within 2, 3, 4, 5, 6, 7, 8, 9, 10 or moreplanes. In a further embodiment, multi-axis bending includes flexing(e.g., curving, bending, deflecting) in at least three directions. Forexample, in some embodiments, the sensor is configured to bend and/orflex in 4, 5, 6, 7, 8, 9, 10 or more directions. In further embodiment,the sensor is configured and arranged without preferred bending pointsand/or locations along its in vivo portion. Accordingly, in theseembodiments, the sensor is configured and arranged for multi-axisbending at any point along the length of the sensor's in vivo portion(e.g., non-preferential bending). In some embodiments, a sensor withmulti-axis bending does not have a preferred bending radius, therebyallowing substantial bending in 360°. Since movements by the host cancause the sensor to bend, it is believed that multi-axis bending extendssensor lifetime (e.g., by preventing sensor breakage and/or degradation)and affords greater host comfort (e.g., by moving/flexing/bending with,instead of resisting, the host's movements, and/or causing tissuedamage).

Multi-axis bending of the certain embodiments includes a combination ofstrength and flexibility. The material properties of the components ofthe in vivo portion of the sensor (e.g., the elongated conductive body,the conductive core, the insulator and/or the membrane) and/or thegeometry of the in vivo portion of the sensor impart this combination ofstrength and flexibility that enables multi-axis bending to the sensor.Material properties can be described in a variety of ways known in theart. For example, tensile strength is the stress at which a materialbreaks or permanently deforms. Ultimate tensile strength (UTS) is themaximum stress a material can withstand when subjected to tension,compression or shearing, and is the maximum stress on a stress-straincurve created during tensile tests conducted on a sensor. Young'smodulus (E) is a measure of the stiffness of an isotropic elasticmaterial, and can be determined from the slope of a stress-strain curvedescribed above. Yield strength is a measure of the ability to bend andnot snap (e.g., break). Fatigue is a measure of the progressive andlocalized structural damage (e.g., the failure or decay of mechanicalproperties) that occurs when a material is subjected to cyclic loading(e.g., stress). The maximum stress values are less than the ultimatetensile stress limit, and may be below the yield stress limit of thematerial.

Fatigue life is the number of cycles of deformation required to bringabout failure of the test specimen under a given set of oscillatingconditions. Fatigue life can be determined by fatigue testing, such asby testing with a device configured to repeatedly bend, pull, compressand/or twist the device. For example, fatigue-life testing can beperformed on a plurality of sensors and then the tensile strength and/orYoung's modulus mathematically determined from data collected during thesensor testing. For example, sensors to be tested can include pre-bentelbows at a predetermined angle, such as but not limited to into a 10,20, 30, 40, 50, 60, 70 or 80-degree elbows, wherein the elbows have abend radius of about 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045 or0.05-inches. Using a fatigue-testing machine (e.g., via a BoseElectroForce® 3200 fatigue-testing unit, Bose Corporation, Eden Prairie,Minn., USA), the elbows can be repeatedly pulled open and/or pushedclosed a predetermined amount, such as but not limited to 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11, 12, 13, 14, or 15-mm or more, and/or through aplurality of deflection ranges, such as but not limited to at a cyclefrequency of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 70, 18, 19 or 20 Hertz. For example, a peak-to-peak deflection of4-mm means that the elbow was pushed in the closed direction 2-mm fromits initial condition, as well as pulled open 2-mm from its initialcondition. The number of cycles (of pulling/pushing) to failure of thedevice (e.g., breaking, buckling, cracking, fraying) can be counted. Inone exemplary embodiment, 60° elbows having a bend radius of about0.025-inches (e.g. bent sensors) can withstand at least about5,000-10,000 cycles of 5-mm peak-to-peak displacement. In anotherexemplary embodiment, the elbows can withstand at least about10,000-70,000 cycles of 4-mm peak-to-peak displacement. In anotherexemplary embodiment, the elbows can withstand at least about1,000,000-10,000,000 cycles of 2-mm peak-to-peak displacement. Inanother exemplary embodiment, the elbows can withstand at least about100,000-600,000 cycles of 3-mm peak-to-peak displacement.

These data (above) can be used to calculate the sensor's tensilestrength, Young's modulus, and the like, as is understood by one skilledin the art. In some embodiments, the sensor is configured for multi-axisbending to an angle of at least about 60°, 70°, 80°, 90°, 100°, 110° or120° or more. In some embodiments, a sensor with multi-axis bending doesnot have a preferred bending radius, thereby allowing substantialbending in 360° about the sensor's longitudinal axis. In someembodiments, the sensor is configured and arranged such that theultimate tensile strength of the elongated conductive body is from aboutless than about 80, 80, 90, 100, 110, 120, 130, 140 or 150 kPsi (551MPa) to about 160, 170, 180, 190, 200, 210, 220 or 500 kPsi (1517 MPa)or more. In some embodiments, the Young's modulus of the sensor is frommore than about 165, 165, 170, 175, 180, 185 or 190 GPa to less thanabout 195, 200, 205, 210, 215 or 220 GPa. In some embodiments, the yieldstrength of the elongated conductive body (e.g., the sensor, conductivecore) is at least about 70, 100, 150, 200, 250, 300, 350, 400, 450, 500,600, 700, 800, 900, 1000, 1250, 1500, 1750, 2000, 2250, 2500, 2750, or3000 MPa or more. In some embodiments, the fatigue life of the sensor isat least about 1,000, 2,000, 3,000, 4,000, or 5,000 cycles or more whenthe sensor is pre-bent into an elbow comprising a bend angle of at least60° and a bend radius of about 0.05-inches or less. In some embodiments,the fatigue life of the sensor is at least 1,000 cycles of flexing offrom about 28° to about 110° and a bend radius of about 0.125-inches.

The analyte sensors (e.g., electrodes and membrane systems) of someembodiments are coaxially and/or concentrically formed. Namely, theelectrodes (e.g., elongated conductive bodies) and/or membrane systemsall share the same central axis. While not wishing to be bound bytheory, it is believed that a coaxial design of the sensor enables asymmetrical design without a preferred bend radius. In contrast toconventional sensors comprising a substantially planar configurationthat can suffer from regular bending about the plane of the sensor, thecoaxial design of the certain embodiments do not have a preferred bendradius and therefore are not subject to regular bending within and/orabout a particular plane (which can cause fatigue failures and thelike). However, non-coaxial sensors can be implemented with the sensorsystem of the some embodiments.

In addition to the above-described advantages, the coaxial sensor designof some embodiments enables the diameter of the connecting end of thesensor (proximal portion) to be substantially the same as that of thesensing end (distal portion). For sensors configured and arranged forimplantation into a host's circulatory system, this configurationenables the protective slotted sheath to insert the sensor into acatheter and subsequently slide back over the sensor and release thesensor from the protective slotted sheath, without complexmulti-component designs. For sensors configured for transcutaneousimplantation, this configuration enables a needle to implant the sensorand then slide over the sensor when the needle is withdrawn.

FIG. 1B is a schematic illustrating an elongated conductive body 102(also referred to as the “elongated body”) in one embodiment, whereinthe elongated conductive body is formed from at least two materialsand/or layers of conductive material, as described in greater detailelsewhere herein. In some embodiments, the term “electrode” can be usedherein to refer to the elongated conductive body, which includes theelectroactive surface that detects the analyte. In some embodiments, theelongated conductive body provides an electrical connection between theelectroactive surface (e.g., working electrode) and sensor electronics(not shown). In certain embodiments, each electrode (e.g., the elongatedconductive body, on which the electroactive surface is located) isformed from a fine wire with a diameter of from about 0.001 inches orless to about 0.01 inches or more, for example, and is formed from,e.g., a plated insulator, a plated wire, or bulk electrically conductivematerial. For example, in some embodiments, the wire and/or elongatedconductive body used to form a working electrode is about 0.002, 0.003,0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025,0.03, 0.035, 0.04 or 0.045 inches in diameter.

In some embodiments, the working electrode (e.g., the elongatedconductive body including an electroactive surface) comprises a wireformed from a conductive material, such as platinum, platinum-iridium,palladium, graphite, gold, carbon, conductive polymer, alloys, or thelike

In some embodiments, the working electrode is formed of platinum-iridiumor iridium wire. In general, platinum-iridium and iridium materials aregenerally stronger (e.g., more resilient and less likely to fail due tostress or strain fracture or fatigue). While not wishing to be bound bytheory, it is believed that platinum-iridium and/or iridium materialscan facilitate fabrication of a wire with a smaller diameter to furtherdecrease the maximum diameter (size) of the sensor (e.g., in vivoportion). Advantageously, with respect to intravascularly-implantedsensors, a smaller sensor diameter both reduces the risk of clot orthrombus formation (or other foreign body response) and allows the useof smaller catheters.

Referring to FIG. 1B, in some embodiments, the elongated conductive body102 comprises at least two concentric layers (e.g., a compositestructure). In a further embodiment, the elongated conductive bodycomprises a core 110 and a first layer 112. The core is formed from oneof the at least two materials referred to above. For example, the corecan be formed of a polymer, a metal, an alloy and the like. In someembodiments, the core is formed from a conductive polymer, such as butnot limited to polyaniline and polypyrrole. In some embodiments, aconductive material is added to (e.g., mixed with and/or applied to) anon-conductive polymer, whereby the polymer core is rendered conductive.For example, in some embodiments, one or more conductive metals (e.g.,carbon, gold, platinum, iridium, etc.), such as but not limited toparticles, can be mixed with the uncured polymer, which can be formedinto the core. Alternatively, the core can comprise an inner core and anouter core, in some embodiments. For example, platinum, iridium or goldparticles can be ion-implanted on the surface of a polymer inner core,such that the particles form an outer core. For example, a polymerfilament fiber can be ion-implanted with gold, such that the treatedfilament fiber is conductive. In some embodiments, the core is formedfrom a metal, such as but not limited to at least one of stainlesssteel, tantalum, titanium and/or an alloy thereof. For example, in oneembodiment, the core is formed of an extruded stainless steel, tantalum,titanium and/or an extruded alloy. In some embodiments, the material ofthe core is processed to provide the strength and flexibility necessaryfor multi-axis bending. Processing the metal changes its properties,such as but not limited to by compressing and/or rearranging the metal'scrystalline lattice. For example, tempering can make a metal lessbrittle and more springy; hardening can make a metal hold its shapebetter. Accordingly, in certain embodiments, the core is formed of ametal that has been processed to provide the requisite combination ofstrength and flexibility (e.g., an ultimate tensile strength of fromabout less than 80, 80, 90, 100, 110, 120, 130, 140 or 150 kPsi (551MPa) to about 160, 170, 180, 190, 200, 210, 220 or 500 kPsi (3297 MPa))or more. For example, in some embodiments, the core is formed from ametal that has been annealed, tempered, normalized, hardened,work-hardened, full-processed, case hardened, draw air hardened, coldworked and/or the like, to render it more stiff. In one embodiment, thecore is formed from full-processed platinum. In another embodiment, thecore is formed from work-hardened platinum-iridium.

In some embodiments, the surface of the elongated conductive body and/orthe core is treated to remove initiation sites (e.g., locations/pointsof irregularity, where sensor breaking tends to begin), to smooth and/orclean the surface, to prepare it for application of the next material,and/or the like. Suitable treatments include but are not limited toelectro-polishing, etching, application of a tie layer,electro-deposition, and electrostatic deposition.

In some embodiments, the elongated conductive body (and/or the core,and/or the sensor) is wire-shaped. However, the wire-shape can includeone of a variety of cross-sectional shapes, such as but not limited to acircle, an oval, a rectangle, a triangle, a cross, a star, a cloverleaf,an X-shape, a C-shape, an irregular or other non-circular configuration,and the like. The elongated conductive body includes a diameter and/or asmallest dimension (e.g., width) of about less than 0.002, 0.002, 0.003,0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025,0.03, 0.035, 0.04 or 0.045 inches or more in diameter. The elongatedconductive body can be provided as a reel and/or extended lengths thatare subsequently processed and/or singularized into individual sensorlengths.

In some embodiments, the elongated conductive body 102 comprises a firstlayer 112 applied to a core 110. In some embodiments, the first layer isapplied to the core such that they are electrically connected (e.g., inelectrical contact, such that a current can pass therebetween). Thefirst layer can be formed of a variety of conductive materials, such asbut not limited to at least one of platinum, platinum-iridium, gold,palladium, iridium, graphite, carbon, conductive polymers and an alloy.In certain embodiments, the first layer is relatively thin, such as butnot limited to a thickness of from about less than 50, 50, 55, 60, 65,70, 75, 80, 85, 90, 95 or 100 micro-inches to about 125, 150, 175, 200,225, 250, 275 or 300 micro-inches, or thicker. As is described elsewhereherein with greater detail, at least a portion of the surface of thefirst layer provides the sensor's electroactive surface (e.g., workingelectrode). For example, as described herein, in some embodiments, theelectroactive surface is exposed through a window formed in theinsulator. In some embodiments, the surface of the applied first layeris treated prior to application of membrane materials, such as tooptimize the surface for membrane attachment and for function as anelectroactive surface. For example, the surface can be cleaned,smoothed, etched, and the like. Advantageously, forming the conductivecore of an inexpensive yet strong and flexible inner body with a thinlayer of the costly electroactive surface material enables a substantialreduction in material costs.

In some embodiments, a conductive paste comprising a mixture of material(e.g., an ink) and an enzyme (e.g., glucose oxidase) may be applied tothe layer surrounding the core, or applied directly to the core. Theconductive paste may also include an interference reducing substance,mediator, or diffusion limiting polymers. Use of the conductive thepaste may reduce or eliminate the need for certain membrane layers(e.g., the enzyme layer).

The first layer 112 can be applied to the core 110 using a variety ofmanufacturing methods. For example, in some embodiments, the first layeris co-extruded with the core using known techniques, such as but notlimited to metal-on-metal or metal-on-polymer extrusion techniques. Someuseful co-extrusion techniques are described in U.S. Pat. No. 7,416,802,U.S. Pat. No. 7,268,562, U.S. Pat. No. 7,153,458, U.S. Pat. No.7,280,879, U.S. Pat. No. 5,324,328 and U.S. Pat. No. 6,434,430. In oneexemplary embodiment, a stainless steel inner body is co-extruded with aplatinum first layer, such as but not limited to through a die, to forma thin reel of 0.005-inch diameter wire having a stainless steel corewith a 100-micro-inch layer of platinum thereon.

In some embodiments, the first layer 112 is applied to the core 110(which, in some embodiments, is pre-treated as described above) using athin film or thick film technique (e.g., spraying, electro-depositing,vapor-depositing, dipping, spin coating, sputtering, evaporation,printing or the like). For example, in one embodiment, the first layeris applied by dipping the core into a solution of the first layermaterial and drawing out the core at a speed that provides theappropriate first layer thickness. However, any known thin or thick filmmethod can be used to apply the first layer to the core, as will beappreciated by one skilled in the art. Some examples of thin and/orthick film manufacturing techniques can be found in U.S. PatentPublication No. 2005-0181012-A1, U.S. Patent Publication No.2006-0036143-A1, U.S. Patent Publication No. 2007-0163880-A1, U.S.Patent Publication No. 2006-0270923-A1, U.S. Patent Publication No.2007-0027370-A1, U.S. Patent Publication No. 2006-0015020-A1, U.S.Patent Publication No. 2006-0189856-A1, U.S. Patent Publication No.2007-0197890-A1, U.S. Patent Publication No. 2006-0257996-A1, U.S.Patent Publication No. 2006-0229512-A1, U.S. Patent Publication No.2007-0173709-A1, U.S. Patent Publication No. 2006-0253012-A1, U.S.Patent Publication No. 2006-0195029-A1, U.S. Patent Publication No.2008-0119703-A1, U.S. Patent Publication No. 2008-0108942-A1, and U.S.Patent Publication No. 2008-0200789-A1.

In some embodiments the first layer 112 is deposited onto the core 110.For example, in some embodiments, the first layer is plated (e.g.,electroplated) onto the core. In one exemplary embodiment, a thin layerof platinum is plated onto a tantalum core by immersing the inner bodyin a platinum-containing solution and applying a current to the innerbody for an amount of time, such that the desired thickness of platinumfirst layer is generated and/or achieved. Description of depositionmethods and devices therefore can be found in U.S. Pat. No. 7,427,338,U.S. Pat. No. 7,425,877, U.S. Pat. No. 7,427,560, U.S. Pat. No.7,351,321 and U.S. Pat. No. 7,384,532.

In still other embodiments, the core 110 is embedded in insulator and aworking electrode body 112 is attached, such that the core and theworking electrode body are electrically (e.g., functionally, operably)connected, such as described with reference to FIGS. 3A and 3B. Forexample, in some embodiments, a working electrode body is formed as afoil that is attached to the core, such as with adhesive, welding and/oran intermediate layer of conductive material to provide adhesion betweenthe core and the working electrode body material (e.g., at tie layer).In some embodiments, multiple layers are applied on top of the core. Insome embodiments, each layer possesses a finite interface with adjacentlayers or together forms a physically continuous structure having agradient in chemical composition. In another embodiment, the workingelectrode body is a C-clip or snap-ring that is attached by compressionabout and/or around the core. In some embodiments, the working electrodebody is attached over a window. In other embodiments, there is nowindow, instead, the working electrode body is configured to pierce theinsulator and to physically contact the underlying core, such that theworking electrode body and the core are operably connected. In someembodiments, a conductive metal C-clip is attached to the core withadhesive, welding and/or a tie layer. In yet another embodiment, anadhesive is attached to the core, followed by wrapping a conductive foilthere-around.

The elongated conductive body 102 can be manufactured using a variety ofmanufacturing techniques. In some embodiments, the first layer 112 isapplied to the core 110 in a substantially continuous process. Forexample, in some embodiments, the manufacturing of the elongatedconductive body involves a reel-to-reel process. In other embodiments, asheet-fed technique is used. In some embodiments, application of thefirst layer to the core can be by either a semi-automated orfully-automated process. Automation of some or all manufacturing stepsgenerally requires the use of one or more machines, such as roboticdevices, that are configured and arranged to perform the manufacturingstep(s). In some embodiments, one manufacturing step can be automated,such as production of the elongated conductive body. However, in otherembodiments, two or more of the manufacturing steps can be automated.For example, a device can be configured to perform two or more of thesteps, or two or more devices can perform the steps. In someembodiments, when multiple devices are used, the devices are connected,coupled together, interconnected, and linked functionally and/orphysically. For example, in some embodiments, the product of one deviceis fed directly into the next device, and so on. In one exemplaryembodiment, a reel of previously manufactured core, such as astainless-steel, tantalum or titanium wire, can be fed substantiallycontinuously through a device configured to electroplate the core withplatinum, gold, carbon or the like, such that a reel of plated wire isgenerated. For example, a manufacturing device and/or system can beconfigured to automatically co-extrude stainless-steel and platinum togenerate/produce a reel of wire-shaped elongated conductive bodycomprising a stainless-steel core and platinum first layer. Examples ofcontinuous manufacturing processes can be found in U.S. Pat. No.6,103,033, U.S. Pat. No. 5,879,828, U.S. Pat. No. 5,714,391, U.S. Pat.No. 7,429,552, U.S. Pat. No. 7,402,349 and U.S. Pat. No. 7,387,811.

In a further embodiment, the first layer comprises an electroactivesurface (e.g., the portion exposed through the window 106). The exposedelectroactive surface of the first layer is the working electrode, insome embodiments. For example, if the sensor is an enzymaticelectrochemical analyte sensor, the analyte enzymatically reacts with anenzyme in the membrane covering at least a portion of the electroactivesurface, which can generate electrons (e⁻) that are detected at theelectroactive surface as a measurable electronic current. For example,in the detection of glucose wherein glucose oxidase produces hydrogenperoxide as a byproduct, hydrogen peroxide reacts with the surface ofthe working electrode producing two protons (2H⁺), two electrons (2e⁻)and one molecule of oxygen (O₂), which produces the electronic currentbeing detected.

As previously described with reference to FIG. 1A and as shown in FIG.1C, an insulator 104 is disposed on (e.g., located on, covers) at leasta portion of the elongated conductive body 102. In some embodiments, thesensor is configured and arranged such that the elongated body includesa core 110 and a first layer 112, and a portion of the first layer isexposed via window 106 in the insulator. In other embodiments, thesensor is configured and arranged such that the elongated body includesa core embedded in an insulator, and a portion of the core is exposedvia the window in the insulator. For example, in some embodiments, theinsulating material is applied to the elongated body (e.g., screen-,ink-jet and/or block-printed) in a configuration designed to leave aportion of the first layer's surface (or the core's surface) exposed.For example, the insulating material can be printed in a pattern thatdoes not cover a portion of the elongated body. In another example, aportion of the elongated body is masked prior to application of theinsulating material. Removal of the mask, after insulating materialapplication, exposes the portion of the elongated body.

In some embodiments, the insulating material 104 comprises a polymer,for example, a non-conductive (e.g., dielectric) polymer. Dip-coating,spray-coating, vapor-deposition, printing and/or other thin film and/orthick film coating or deposition techniques can be used to deposit theinsulating material on the elongated body and/or core. For example, insome embodiments, the insulating material is applied as a layer of fromabout less than 5, 5, 10 or 15-microns to about 20, 25, 30 or 35-micronsor more in thickness. In some embodiments, the insulator is applied as asingle layer of material. In other embodiments, the insulator is appliedas two or more layers, which are comprised of either the same ordifferent materials. In some embodiments, the insulating materialcomprises at least one of polyurethane, polyimide and parylene. In oneembodiment, the insulating material comprises parylene, which can be anadvantageous polymer coating for its strength, lubricity, and electricalinsulation properties. Generally, parylene is produced by vapordeposition and polymerization of para-xylylene (or its substitutedderivatives). However, any suitable insulating material, such as but notlimited to a dielectric ink, paste or paint, can be used, for example,fluorinated polymers, polyethyleneterephthalate, polyurethane,polyimide, other nonconducting polymers, or the like. In someembodiments, glass or ceramic materials can also be employed. Othermaterials suitable for use include surface energy modified coatingsystems such as are marketed under the trade names AMC18, AMC148,AMC141, and AMC321 by Advanced Materials Components Express ofBellafonte, Pa. In some alternative embodiments, however, the conductivecore may not require a coating of insulator. In certain embodiments, theinsulating material defines an electroactive surface of the analytesensor (e.g., the working electrode). For example, in some embodiments asurface of the conductive core (e.g., a portion of the first layer 112)either remains exposed during the insulator application or a portion ofapplied insulator is removed to expose a portion of the conductivecore's surface, as described above.

In some embodiments, in which the sensor has an insulated elongatedbody, a portion of the insulating material is stripped or otherwiseremoved, for example, by hand, excimer lasing, chemical etching, laserablation, grit-blasting (e.g., with sodium bicarbonate or other suitablegrit), or the like, to expose the electroactive surfaces. In oneexemplary embodiment, grit blasting is implemented to expose theelectroactive surface(s), for example, by utilizing a grit material thatis sufficiently hard to ablate the polymer material yet alsosufficiently soft so as to minimize or avoid damage to the underlyingmetal electrode (e.g., a platinum electrode). Although a variety of“grit” materials can be used (e.g., sand, talc, walnut shell, groundplastic, sea salt, and the like), in some embodiments, sodiumbicarbonate is an advantageous grit-material because it is sufficientlyhard to ablate, e.g., a parylene coating without damaging, e.g., anunderlying platinum conductor. One additional advantage of sodiumbicarbonate blasting includes its polishing action on the metal as itstrips the polymer layer, thereby eliminating a cleaning step that mightotherwise be necessary. In some embodiments, the opening in theinsulator, through which the surface of the first layer is exposed, isreferred to as a “window” 106.

Due to the small sizes of the sensors in some embodiments, it can bedifficult to precisely remove the insulator over one insulatedconductive core without affecting, and possibly removing, the insulatorover an adjacent conductive core or over other parts of the sensor.However, in some embodiments, the insulator is configured such that theprecision of laser ablation is substantially improved. For example, insome embodiments, the insulator is configured such that two differenttypes of lasers can be used to ablate separate portions of theinsulator. For example, if the insulators of two elongated bodies aredifferent materials (i.e. one is polyurethane and another is TEFLON® oranother type of polytetrafluoroethylene), then it is possible toselectively ablate the insulator off of one of the elongated bodies andto not remove insulator from the other elongated body in the same regionof the sensor. In some embodiments, the two insulation materials requiredifferent laser parameters for optimal ablation, such that a first lasersetup could be used to ablate a first material but not the secondmaterial, and a second laser setup could be used to ablate a secondmaterial but not the first material. In another example, for a sensorcontaining two elongated bodies, the insulator covering one elongatedbody can be configured for laser ablation with an ultraviolet laser(e.g., using a wavelength of about 200 nm), and the other elongated bodycan be configured for laser ablation with an infrared laser (e.g., usinga wavelength of about 1000 nm). In another embodiment, the insulatormaterials are selected such that the insulator of a first elongated bodyrequires a substantially higher laser power to be ablated than theinsulator of a second elongated body. For example, the insulator overthe two elongated bodies can be the same, except that the insulator ofthe first elongated body is thicker than the insulator of the secondelongated body. In another example, the insulator on each of theelongated bodies has a different thickness, such that a single laser isused to remove the insulator over both cores, except that the window inthe thinner insulator is formed more quickly than the window in thethicker insulator. For example, the insulator of one elongated body canbe from about 0.0001 inches to about 0.0003 inches in thickness, and theinsulator of one elongated body can be from about 0.0008 inches to about0.0010 inches in thickness. In yet another example, a colorant can beadded to the insulator of one of the elongated bodies, to modify theamount of energy that is absorbed from the laser. For example, adding adark colorant or other absorptive material to the first insulator butnot the second insulator can cause the first insulator to absorb muchmore energy of the laser than the non-colored second insulator. In thisway, a small amount of laser energy would ablate one wire but not theother, but a large amount of laser energy would ablate both. As isunderstood by one skilled in the art, the setup of the laser can beadjusted, to fine-tune the insulator removal process. For example, thelaser pulse width and power level can be adjusted to modify and/ormodulate the amount of insulator removed, the rate of removal, and/orthe like. This principle can be used for assemblies (e.g., sensors) ofthree or more elongated bodies (e.g., cores, wires). The same principlemay be applied to chemical ablation, where different solvents arerequired for the different insulation layers such that they can beselectively ablated. The same principle may also be used with plasmaablation, where different plasma settings or amounts of energy arerequired to ablate the different materials.

The electroactive surface of the working electrode is exposed byformation of a window 106 in the insulator 104. The electroactive window106 of the working electrode is configured to measure the concentrationof an analyte. For example, in an enzymatic electrochemical sensor fordetecting glucose, the working electrode measures the hydrogen peroxideproduced by an enzyme catalyzed reaction of the analyte being detectedand creates a measurable electronic current. For example, in thedetection of glucose wherein glucose oxidase (GOX) produces hydrogenperoxide as a byproduct, hydrogen peroxide reacts with the surface ofthe working electrode producing two protons (2H⁺), two electrons (2e⁻)and one molecule of oxygen (O₂), which produces the electronic currentbeing detected. The sensor can be configured to detect other analytes bysubstituting an enzyme that metabolizes the analyte of interests forGOX, as is understood by one skilled in the art.

In the embodiments illustrated in FIGS. 1A, 1C, 2A, 4B, and 5A through5D, a radial window 106 is formed through the insulating material 104 toexpose a circumferential electroactive surface of the working electrode(e.g., first layer 112). In other embodiments, such as those shown inFIGS. 3A, 3B, 4A, 5B and 5D, a radial or non-radial window 106 is formed(e.g., for electrical connection to the working electrode body 112) byremoving only a portion of the insulating material 104. Additionally, asection of electroactive surface of the reference electrode 114 isexposed, in some embodiments (not shown). For example, the sections ofelectroactive surface can be masked during deposition of an outerinsulating layer and/or etched after deposition of an outer insulatinglayer. In some embodiments, a plurality of micro-windows comprises theelectroactive surface of the working electrode, wherein the sum of themicro-window surface areas is substantially equal to the window 106electroactive surface area. In certain embodiments, the plurality ofmicro-windows are spaced and/or staggered along a length of theconductive core.

In some embodiments, the window 106 (or the working electrode body 112)is sized to provide an electroactive surface (e.g., working electrode)having an area such that the sensor functions in the picoAmp range(e.g., when the analyte is glucose, a sensitivity of from about 1 toabout 300 pA per mg/dL glucose, or a sensitivity of from about 5 toabout 100 pA per mg/dL glucose, or from about 5 to about 25 pA per mg/dLglucose, and or from about 4 to about 7 pA per mg/dL). For an electrodehaving an electroactive surface area of about 0.3 mm², the currentdensity (sensitivity divided by surface area) is or from about 17pA/mg/dL/mm² to about 1000 pA/mg/dL/mm², or from about 3 pA/mg/dL/mm² toabout 83 pA/mg/dL/mm², or 13 pA/mg/dL/mm² to about 23 pA/mg/dL/mm². Insome embodiments, the working electrode has a diameter of from about0.001 inches or less to about 0.01 inches or more, or from about 0.002inches to about 0.008 inches, or from about 0.004 inches to about 0.005inches. The length of the window can be from about 0.1 mm (about 0.004inches) or less to about 2 mm (about 0.078 inches) or more, or fromabout 0.5 mm (about 0.02 inches) to about 0.75 mm (0.03 inches). In suchembodiments, the exposed surface area of the working electrode is fromabout 0.000013 in² (0.0000839 cm²) to about 0.0025 in² (0.016129 cm²)(assuming a diameter of from about 0.001 inches to about 0.01 inches anda length of from about 0.004 inches to about 0.078 inches). The exposedsurface area of the working electrode is selected to produce an analytesignal with a current in the picoAmp range. However, a current in thepicoAmp range can be dependent upon a variety of factors, for examplethe electronic circuitry design (e.g., sample rate, current draw, A/Dconverter bit resolution, etc.), the membrane system (e.g., permeabilityof the analyte through the membrane system), and the exposed surfacearea of the working electrode. Accordingly, the exposed electroactiveworking electrode surface area can be selected to have a value greaterthan or less than the above-described ranges taking into considerationalterations in the membrane system and/or electronic circuitry. Incertain embodiments of a glucose sensor, it can be advantageous tominimize the surface area of the working electrode while maximizing thediffusivity of glucose in order to optimize the signal-to-noise ratiowhile maintaining sensor performance in both high and low glucoseconcentration ranges.

In some embodiments, the exposed surface area (e.g., electroactivesurface) of the working (and/or other) electrode (e.g., conductive core)can be increased by altering the cross-section of the electrode itself.For example, in some embodiments the cross-section of the workingelectrode can be defined by a cross, star, cloverleaf, ribbed, dimpled,ridged, irregular, or other non-circular configuration; thus, for anypredetermined length of electrode, a specific increased surface area canbe achieved (as compared to the area achieved by a circularcross-section). Increasing the surface area of the working electrode canbe advantageous in providing an increased signal responsive to theanalyte concentration, which in turn can be helpful in improving thesignal-to-noise ratio, for example. In some embodiments, application ofthe insulator to the conductive core can be accomplished by asubstantially continuous process, which can be semi- or fully-automated,such as in a manner similar to some methods described forformation/manufacture of the conductive core.

In some embodiments, the sensor 100 further comprises a referenceelectrode 114. The reference electrode 114, which can function as areference electrode alone, or as a dual reference and counter electrode,is formed from silver, silver/silver chloride, or the like. In someembodiments, the reference electrode 114 is juxtapositioned and/ortwisted with or around at least a portion of the sensor. For example, inFIG. 2A, the reference electrode is a silver wire helically twistedand/or wrapped and/or wound around the working electrode. This assemblyof “wires” is then optionally coated or adhered together with aninsulating material, similar to that described above, so as to providean insulating attachment.

In some embodiments, a silver wire is formed onto and/or fabricated intothe sensor and subsequently chloridized to form silver/silver chloridereference electrode. Advantageously, chloridizing the silver wire asdescribed herein enables the manufacture of a reference electrode withoptimal in vivo performance. Namely, by controlling the quantity andamount of chloridization of the silver to form silver/silver chloride,improved break-in time, stability of the reference electrode andextended life has been shown with some embodiments. Additionally, use ofsilver chloride as described above allows for relatively inexpensive andsimple manufacture of the reference electrode.

Referring to FIGS. 1B-1C, in some embodiments, the reference electrode114 comprises a silver-containing material applied over at least aportion of the insulating material 104. In some embodiments, thesilver-containing material is applied using thin film and/or thick filmtechniques, such as but not limited to dipping, spraying, printing,electro-depositing, vapor deposition, spin coating, and sputterdeposition, as described elsewhere herein. For example, a silver orsilver-chloride-containing paint (or similar formulation) is applied toa reel of the insulated conductive core, in one embodiment. In anotherexample, the reel of insulated elongated body (or core) is cut intosingle unit pieces (e.g., “singularized”) and a silver-containing ink ispad printed thereon. In still other embodiments, the silver-containingmaterial is applied as a silver foil. For example, an adhesive can beapplied to an insulated elongated body, around which the silver foil isthen wrapped in. Alternatively, the sensor can be rolled in Ag/AgClparticles, such that a sufficient amount of silver sticks to and/orembeds into and/or otherwise adheres to the adhesive for the particlesto function as the reference electrode. In some embodiments, thesensor's reference electrode includes a sufficient amount of chloridizedsilver that the sensor measures and/or detects the analyte for at leastthree days.

In some embodiments, the sensor is formed from an elongated body 102(e.g., elongated conductive body), such as that shown in FIG. 1B,wherein the elongated body includes a core 110, a first layer 112, aninsulator 104, and a layer of silver-containing material 114. In someembodiments, such as that shown in FIG. 1C, the electroactive surface ofthe elongated body (e.g., also the (electroactive) surface of the firstlayer 112) is exposed by formation of a window 106 through both thesilver-containing material and the insulator. In one exemplaryembodiment, the elongated body of FIG. 1B is provided as an extendedlength on a reel that is singularized into a plurality of pieces havinga length (e.g., less than 0.5, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5,5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13,13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20,20.5, 21, 21.5, 22, 22.5, 23, 23.5 or 24-inch or longer lengths)suitable for a selected sensor configuration. For example, a firstsensor configured for transcutaneous implantation can employ 2.5-inchlengths, while a second sensor configured for transcutaneousimplantation can employ 3-inch lengths. In another example, a firstsensor configured for implantation into a peripheral vein of an adulthost can employ a 3-inch length, while a second sensor configured forimplantation into a central vein of an adult host can employ a 12-inchlength. The window is formed on each sensor, such as by scraping and oretching a radial window through the silver-containing material and theinsulator such that the platinum surface is exposed (e.g., theelectroactive surface of the “working electrode”). In some embodiments,a reel of elongated body is singularized and then the windows areformed. In other embodiments, the windows are formed along the length ofthe reel of elongated body, and then later singularized. In a furtherembodiment, additional manufacturing steps are performed prior tosingularization. A membrane 108 is applied to the exposed electroactivesurface (e.g., the working electrode) defined by the edges of thewindow, such that the electroactive surface can function as the workingelectrode of the sensor to generate a signal associated with an analyte(e.g., when the sensor is in contact with a sample of a host).Alternative manufacturing techniques and/or sequences of steps can beused to produce sensors having the configuration shown in FIG. 1C, suchas but not limited to masking a portion of the elongated body (or core)prior to application of the insulator and the silver-containingmaterial.

FIG. 1B is an illustration showing layers cut away, but in thefabrication process the material typically obtained has all layersending at a tip. A step of removing layers 104 and 114 can be performedso as to form window(s). FIG. 1D illustrates the results of thisremoval/cutting away process through a side-view/cross-section. Theremoval process can be accomplished by the methods already described orother methods as known in the art. In one embodiment the removal step isconducted, e.g., by laser skiving, and can be performed in areel-to-reel process on a continuous strand. The removed area can bestepped, for example, by removing different layers by different lengths(FIG. 1D). In such a fabrication method involving a continuous strand,the sensors can be singularized after the removal step. In someembodiments, if the core is a metal, an end cap may be employed, e.g.,by dipping, spraying, shrink tubing, crimp wrapping, etc., an insulatingor other isolating material onto the tip. If the core is a polymer(e.g., hydrophobic material), an end cap may be omitted. For example, inthe sensor depicted in FIG. 1D, an end cap 120 (e.g., of a polymer or aninsulating material) or other structure may be provided over the core(e.g., if the core 110 is not insulating). FIG. 1E can be considered tobuild on a general structure as depicted in FIG. 1B, in that two or moreadditional layers are added to create one or more additional electrodes.Methods for selectively removing two or more windows to create two ormore electrodes can also be employed. For example, by adding anotherconductive layer 122 and insulating layer 124 under a referenceelectrode layer 114, then two electrodes (first and second workingelectrodes) can be formed, yielding a dual electrode sensor. The sameconcept can be applied to create, a counter electrode, electrodes tomeasure additional analytes (e.g., oxygen), and the like, for example.FIG. 1F illustrates a sensor having an additional electrode 122 (ascompared to FIGS. 1B-1D), wherein the windows are selectively removed toexpose working electrodes 112, 122 in between a reference electrode(including multiple segments) 114, with a small amount of insulator 104,124 exposed therebetween. FIG. 1G illustrates another embodiment,wherein selective removal of the various layers is stepped to expose theelectrodes 112, 122 and insulators 104, 124 along the length of theelongated body.

In some embodiments, the silver-containing material is applied to thesensor (e.g., the insulated conductive core) in a substantiallycontinuous process, such as described elsewhere herein. Accordingly, insome embodiments, the silver-containing material is applied in afully-automated process. In other embodiments, the silver-containingmaterial is applied in a semi-automated process.

Referring to FIGS. 2A to 2B, in some embodiments, the sensor can beconfigured similarly to the continuous analyte sensors disclosed inco-pending U.S. Patent Publication No. 2007-0197889-A1. The sensorincludes a distal portion 202, also referred to as the in vivo portion,adapted for implantation into a host, and a proximal portion 204, alsoreferred to as an ex vivo portion, adapted to operably connect to thesensor electronics. In certain embodiments, the sensor includes two ormore electrodes: a working electrode (e.g., the electroactive surface ofthe elongated conductive body 102/first layer 112) and at least oneadditional electrode (e.g., electroactive surface), which can functionas a counter electrode and/or reference electrode, hereinafter referredto as the reference electrode 114. In this embodiment, an insulator 104is deposited over the conductive core. A radial window 106 is formedthrough the insulator, such that the working electrode/electroactivesurface is exposed. The reference electrode is formed from a silver wirehelically wound/wrapped around at least a portion of the sensor. Thesilver wire can be chloridized either before and/or after application tothe sensor. The insulator, which is disposed between the elongatedconductive body and reference electrode, provides electrical insulationtherebetween. A membrane system may be deposited over the electrodes,such as described in more detail below with reference to FIGS. 6A-C.

Although the embodiments of FIGS. 2A to 2B illustrate one electrodeconfiguration including one bulk metal wire helically wound aroundanother bulk metal wire, other electrode configurations are alsocontemplated. In an alternative embodiment, the working electrodecomprises a tube with a reference electrode disposed or coiled inside,including an insulator therebetween. Alternatively, the referenceelectrode comprises a tube with a working electrode disposed or coiledinside, including an insulator therebetween. In another alternativeembodiment, a polymer (e.g., insulating) rod is provided, wherein theelectrodes are deposited (e.g., electro-plated) thereon. In yet anotheralternative embodiment, a metallic (e.g., steel) rod is provided, coatedwith an insulating material, onto which the working and referenceelectrodes are deposited. In yet another alternative embodiment, one ormore working electrodes are helically wound around a referenceelectrode.

FIG. 3A is a perspective schematic illustrating an alternativeembodiment of the analyte sensor, wherein the working electrode body 112is formed separately from the core 110. FIG. 3B is a cross-sectionalview of the sensor of FIG. 3A. In this embodiment, the core is coatedwith and/or embedded in the insulator 104. A portion of the insulator isremoved to provide a window 106 therein. The working electrode body isapplied to the core, such that the working electrode body and the coreare in electrical contact (e.g., functionally connected). In someembodiments, the working electrode body is formed as a C-clip that isattached over the window. In this embodiment, the interior surface ofthe C-clip (e.g., working electrode body) makes either direct electricalcontact with the exposed surface of the core, such as by the two memberstouching, or via indirect contact through an intervening mediaplaced/applied on the core and/or the C-clip prior to connection (e.g.,an electrically conductive adhesive, gel, paint or other media). Inother embodiments, the working electrode body is applied to the exposedsurface of the core as a conductive ink, paint or paste, which issubsequently cured and/or dried. For example, in one embodiment, an inkcontaining platinum particles is printed into the window. In anotherembodiment, a conductive material, such as a liquid metal, is applieddirectly to the exposed surface of the core. In another embodiment, nowindow is formed. Rather, the working electrode body is configured topierce the insulator and to make physical (e.g., electrical, functional)contact with the core. In some embodiments, such as that shown in FIG.7, the reference electrode is printed on the elongated body, usingmethods known in the art. For example, in some embodiments, thereference electrode is a silver-containing ink, paint or paste, such asbut not limited to a silver-containing polymer, that is printed on theelongated body using thin-film and/or thick-film printing techniques.

The electrochemical analyte sensors described herein are configured togenerate a signal associated with a concentration of the analyte in thehost. The sensors provide at least one working electrode and at leastone reference electrode. The output signal is typically a raw datastream that is used to provide a useful value of the measured analyteconcentration in a host to the patient or doctor, for example. Theanalyte sensors of certain embodiments may further measure at least oneadditional signal. For example, in some embodiments, the additionalsignal is associated with the baseline and/or sensitivity of the analytesensor, thereby enabling monitoring of baseline and/or sensitivitychanges that may occur in a continuous analyte sensor over time.Additionally or alternatively, multiple working electrodes can allow formeasurement of multiple analytes, which may also allow for improvedaccuracy in the measurement of glucose, or allow for detection ofcertain conditions that can affect sensor accuracy.

In some embodiments, the sensor comprises a second elongated conductivebody 102 (or a core that can be electrically connected with a workingelectrode body). In some embodiments, the second elongated conductivebody is configured as a counter electrode. In other embodiments, asensor comprising a second elongated conductive body (or core) isconfigured and arranged as a second working electrode, as describedbelow. In some embodiments, the sensor comprises at least threeelongated conductive bodies (or cores). The insulating material 104covers at least a portion of each of the first and second elongatedconductive bodies (or cores). In some embodiments, the insulatingmaterial covering at least a portion of each of the first and secondelongated conductive bodies (or cores) is unitary, such that theinsulating material covers at least a portion of both the first andsecond elongated conductive bodies (or cores). For example, in someembodiments, the elongated conductive bodies (or cores) are disposed(e.g., embedded, located) within the same insulator.

FIG. 3B is a schematic illustrating a cross-section of an analyte sensorin one embodiment, in which an insulated conductive body includes aplurality of conductive cores 110A, 110B, and 110C located (e.g.,embedded) in the insulator 104. A surface of core 110B is exposed bywindow 106, and a working electrode body 112 is applied to the exposedsurface of the core. FIG. 3B shows a single working electrode body.However, each core can have a working electrode body attached thereto(e.g., in electrical contact with it). Accordingly, in some embodiments,a completed sensor includes one, two, or three working electrode bodies.Similarly, if the insulated conductive body includes more than threecores, then the completed sensor manufactured from that insulatedconductive body can include a corresponding number of elongatedconductive bodies.

In the embodiment shown in FIG. 3B, the working electrode body is aformed structure and/or body (e.g., a C-clip, wire or foil) that isattached at the window. In an alternative embodiment, the workingelectrode body comprises an amorphous material (e.g., an ink, paint orpaste) applied to the exposed surface (e.g., through the window) andsubsequently cured. In another alternative embodiment, the workingelectrode body is configured for application over the insulator (e.g.,no window is formed) and to extend through (e.g., pierce, intersect) theinsulator, such that at least the ends of the working electrode bodymake electrical contact with the core. In some embodiments, the core andworking electrode body make direct physical contact, such that anelectrical current can pass therebetween. However, a conductiveintermediary, such as a conductive adhesive, gel, lubricant, paint, inkor paste is disposed therebetween, such as to enable current transferfrom one component to the other, or to promote attachments between twoincompatible materials (e.g., that will not readily adhere to eachother). In this embodiment, one, some or all of the inner bodies can beconnected and/or attached to a working electrode body, wherein thesensor includes one, two, three, or more working electrodes. Inembodiments in which two or more windows are formed in the insulator,the windows are staggered along a length of the sensor (e.g., the invivo portion). In other embodiments, the windows are not staggered alongthe length of the sensor. In some embodiments, a silver-containingmaterial is applied over the insulator, to form a reference electrode.In other embodiments, a silver wire is wrapped around the sensor, toform the reference electrode, as described elsewhere herein.

FIG. 4A is a perspective view of the in vivo portion of an analytesensor in another embodiment. In this embodiment, the insulatedelongated body comprises three conductive cores 110A, 110B, 110C locatedin (e.g., embedded in, coated with) the insulator 104. In thisembodiment, a plurality of windows is formed in and/or through theinsulator, such that each window exposes a portion of a core. As anon-limiting example, window 106A is formed in the insulator such that aportion of core 110A is exposed. Similarly, window 106B is formed in theinsulator such that a portion of core 110B is exposed. The windows canbe staggered and/or non-staggered along the longitudinal length of thesensor. In a further embodiment, each conductive core includes an innercore and an outer core, such as described elsewhere herein.

FIG. 4B is a perspective view of the in vivo portion of an analytesensor including an elongated body (e.g., configured and arranged formulti-axis bending) formed of an insulator 104, first and secondconductive cores 110A, 110B embedded in the insulator, and a membrane108. The first conductive core is formed of platinum, platinum-iridium,gold, palladium, iridium, graphite, carbon, a conductive polymer and/oran alloy, and a first window 106A is configured and arranged to exposean electroactive portion of the first conductive core. The secondconductive core is formed of a silver-containing material (e.g., asilver or silver/silver-chloride wire, or a silver-containingwire-shaped a silver-containing material body), and a second window 106Bis configured and arranged to expose an electroactive portion of thesecond conductive core. In some embodiments, instead of a bulk metalwire, the first conductive core comprises an inner core and an outercore. For example, to reduce material costs, the inner core is formed ofa material that is relatively less expensive than platinum, such asstainless steel, titanium, tantalum and/or a polymer, and the outer coreis formed of a material that provides an appropriate electroactivesurface, such as but not limited to platinum, platinum-iridium, gold,palladium, iridium, graphite, carbon, a conductive polymer and/or analloy. In some embodiments, the membrane covers the exposedelectroactive portion of the first conductive core. In a furtherembodiment, the membrane covers the in vivo portion of the sensor. Insome embodiments, a third conductive core is embedded in the insulator.In some embodiments, the third conductive core is configured andarranged as a second working electrode, which can be configured as aredundant working electrode, a non-analyte signal-measuring workingelectrode (e.g., no enzyme as described below), as a counter workingelectrode, to detect a second analyte, and/or the like.

FIG. 4C is a perspective view of the in vivo portion of an analytesensor comprising three insulated conductive bodies, wherein eachinsulated conductive body includes a core (e.g., 110A, 110B and 110C)coated with insulator (e.g., 104A, 104B and 104C). In some embodiments,one or more of the cores is formed of a material that provides theelectroactive surface of the working electrode, such as but not limitedto platinum, platinum-iridium, gold, palladium, iridium, graphite,carbon, a conductive polymer and/or an alloy. However, in someembodiments, one or more of the cores is formed of an inner core and anouter core, wherein a portion of the surface of the outer core providesthe electroactive surface of the working electrode. In still otherembodiments, one or more of the cores is formed of a material thatprovides electrical conduction from the working electrode (e.g., anattached working electrode body) to sensor electronics. Materialssuitable to provide electrical conduction include, but are not limitedto stainless steel, titanium, tantalum and/or a conductive polymer. Insome embodiments, one or more working electrode bodies 112 are disposed(e.g., applied, attached, located) on the cores, as described elsewhereherein. In some embodiments, the cores (e.g., coated with insulator) arebundled together, such as by an elastic band, an adhesive, wrapping, ashrink-wrap or C-clip, as is known in the art. In other embodiments, theinner bodies (e.g., coated with insulator) are twisted, such as into atriple-helix or similar configuration. In one embodiment, two of thecores (e.g., coated with insulator) are twisted together to form atwisted pair, and then a third core (e.g., with insulator) and/orelongated conductive body is twisted around the twisted pair. In someembodiments, the sensor comprises additional cores (e.g., coated withinsulator).

FIG. 5A is a perspective view of the in vivo portion a dual-electrodeanalyte sensor, in one embodiment. In this embodiment, the sensorcomprises first and second bundled elongated bodies (e.g., conductivecores) E1, E2, wherein a working electrode comprises an exposedelectroactive surface of the elongated body, and a reference electrode114, wherein each working electrode comprises a conductive core. Forexample, the first working electrode comprises an exposed portion of thesurface of a first elongated body 102A having an insulating material104A disposed thereon, such that the portion of the surface of theelongated body (e.g., the working electrode) is exposed via a radialwindow 106A in the insulator. In some embodiments, the elongated bodycomprises a core and a first layer, wherein an exposed surface (e.g.,electroactive) of the first layer is the first working electrode. Thesecond working electrode comprises an exposed surface of a second core110B having an insulator 104B disposed thereon, such that a portion ofthe surface of the core is exposed via a radial window 106B in theinsulator. A first layer (not shown) is applied to the exposed surfaceof the second core to form the second working electrode. In thisembodiment, the radial windows are spaced such that the workingelectrodes (e.g., electroactive surfaces) are substantially overlappingalong the length of the sensor. However, in other embodiments, theworking electrodes are spaced such that they are not substantiallyoverlapping along the length of the sensor. In this embodiment, thereference electrode comprises a wire (e.g., Ag/AgCl wire) wrapped aroundthe bundled conductive cores. However, in some embodiments, thereferenced electrode comprises a layer of silver-containing materialapplied to at least one of the conductive cores, such as described withreference to FIG. 1B.

FIG. 5B is a perspective view of the in vivo portion of a dual-electrodeanalyte sensor, in another embodiment. In this embodiment, the first andsecond elongated bodies E1, E2 are twisted into a twisted pair, such asa helix. The reference electrode 114 is then wrapped around the twistedpair.

FIGS. 5C and 5D include views of the in vivo portion of a dual-electrodeanalyte sensor, in additional embodiments. In these embodiments, thefirst and second elongated bodies E1, E2 are bundled together withreference electrode 114. Connectors 502, 530 are configured and arrangedto hold the conductive cores and reference electrode together.Alternatively, instead of connectors 502, a tube 530 or heat shrinkmaterial can be employed as a connector and/or supporting member. Thetubing or heat shrink material may include an adhesive inside the tubeso as to provide enhanced adhesion to the components secured within(e.g., wire(s), core, layer materials, etc.). In such a configuration,the heat-shrink material functions not only as an insulator, but also tohold the proximal ends of the sensor together so as to prevent or reducefatigue and/or to maintain the electrodes together in the event of afatigue failure. In the embodiment depicted in FIG. 5C, the wires neednot be a core and a layer, but can instead comprise bulk materials. Thedistal ends of the sensor can be loose and finger-like, as depicted inFIG. 5C, or can be held together with an end cap. A reference electrodecan be placed on one or more of the first and second elongated bodiesinstead of being provided as a separate electrode, and the first andsecond elongated bodies including at least one reference electrodethereof can be bundled together. Heat shrink tubing, crimp wrapping,dipping, or the like can be employed to bundle one or more elongatedbodies together. In some embodiments, the reference electrode is a wire,such as described elsewhere herein. In other embodiments, the referenceelectrode comprises a foil. In an embodiment of a dual-electrode analytesensor, the first and second elongated bodies can be present as orformed into a twisted pair, which is subsequently bundled with a wire orfoil reference electrode. Connectors, which can also function assupporting members, can be configured and arranged to hold theconductive cores and reference electrode together.

In some embodiments, a dual-electrode sensor is configured and arrangedto detect two analytes and/or configured as plus-enzyme and minus-enzymeelectrodes. In certain embodiments of a dual-electrode analyte sensor,the first working electrode (e.g., the electroactive surface of thefirst elongated body E1) is configured and arranged to generate a firstsignal comprising an analyte component and a baseline, and the secondworking electrode (e.g., the electroactive surface of the secondelongated body E2) is configured and arranged to generate a secondsignal comprising baseline without an analyte component. In one suchdual-electrode system, a first electrode functions as a hydrogenperoxide sensor including a membrane system containing glucose-oxidasedisposed thereon, which operates as described herein. A second electrodeis a hydrogen peroxide sensor that is configured similar to the firstelectrode, but with a modified membrane system (without active enzyme,for example). This second electrode provides a signal composed mostly ofthe baseline signal, b.

In some dual-electrode systems, the baseline signal is (electronicallyor digitally) subtracted from the glucose signal to obtain a glucosesignal substantially without baseline. Accordingly, calibration of theresultant difference signal can be performed by solving the equationy=mx with a single paired measurement. Calibration of the insertedsensor in this alternative embodiment can be made less dependent on thevalues/range of the paired measurements, less sensitive to error inmanual blood glucose measurements, and can facilitate the sensor's useas a primary source of glucose information for the user. U.S. PatentPublication No. 2005-0143635-A1, U.S. Patent Publication No.2007-0027385-A1, U.S. Patent Publication No. 2007-0213611-A1, and U.S.Patent Publication No. 2008-0083617-A1 describe systems and methods forsubtracting the baseline from a sensor signal.

In some alternative dual-electrode system embodiments, the analytesensor is configured to transmit signals obtained from each electrodeseparately (e.g., without subtraction of the baseline signal). In thisway, the receiver can process these signals to determine additionalinformation about the sensor and/or analyte concentration. For example,by comparing the signals from the first and second electrodes, changesin baseline and/or sensitivity can be detected and/or measured and usedto update calibration (e.g., without the use of a reference analytevalue). In one such example, by monitoring the corresponding first andsecond signals over time, an amount of signal contributed by baselinecan be measured. In another such example, by comparing fluctuations inthe correlating signals over time, changes in sensitivity can bedetected and/or measured.

In some embodiments, a substantial portion of the in vivo portion of thesensor is designed with at least one dimension less than about 0.004inches, 0.005 inches, 0.006 inches, 0.008 inches, 0.01 inches, 0.012,0.015, or 0.02 inches. In some embodiments, in which the sensor isconfigured and arranged for implantation into a host vessel, asubstantial portion of the sensor that is in fluid contact with theblood flow is designed with at least one dimension less than about0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.008, 0.01, 0.012, or 0.015,inches. As one exemplary embodiment, a sensor such as described in moredetail with reference to FIGS. 5A to 5E is formed from a 0.004 inchconductive wire (e.g., platinum) for a diameter of about 0.004 inchesalong a substantial portion of the sensor (e.g., in vivo portion orfluid contact portion). As another exemplary embodiment, a sensor suchas described in more detail with reference to FIGS. 5A to 5E is formedfrom a 0.004 inch conductive wire and vapor deposited with an insulatormaterial for a diameter of about 0.005 inches along a substantialportion of the sensor (e.g., in vivo portion or fluid contact portion),after which a desired electroactive surface area can be exposed. In theabove two exemplary embodiments, the reference electrode can be locatedremote from the working electrode (e.g., formed from the conductivewire). While the devices and methods described herein are suitable foruse within the host's blood stream, one skilled in the art willrecognize that the systems, configurations, methods and principles ofoperation described herein can be incorporated into other analytesensing devices, such as but not limited to transcutaneous devices,subcutaneous devices, and wholly implantable devices such as describedin U.S. Patent Publication No. 2006-0016700-A1.

In addition to the embodiments described above, the sensor can beconfigured with additional working electrodes as described in U.S.Patent Publication No. 2005-0143635-A1, U.S. Pat. No. 7,081,195, andU.S. Patent Publication No. 2007-0027385-A1. For example, in oneembodiment have an auxiliary working electrode, wherein the auxiliaryworking electrode comprises a wire formed from a conductive material,such as described with reference to the glucose-measuring workingelectrode above. The reference electrode, which can function as areference electrode alone, or as a dual reference and counter electrode,is formed from silver, silver/silver chloride, and the like.

In some embodiments, the electrodes are juxtapositioned and/or twistedwith or around each other; however other configurations are alsopossible. In one example, the auxiliary working electrode and referenceelectrode can be helically wound around the glucose-measuring workingelectrode. Alternatively, the auxiliary working electrode and referenceelectrode can be formed as a double helix around a length of theglucose-measuring working electrode. The assembly of wires can then beoptionally coated together with an insulating material, similar to thatdescribed above, in order to provide an insulating attachment. Someportion of the coated assembly structure is then stripped, for exampleusing an excimer laser, chemical etching, and the like, to expose thenecessary electroactive surfaces. In some alternative embodiments,additional electrodes can be included within the assembly, for example,a three-electrode system (including separate reference and counterelectrodes) as is appreciated by one skilled in the art.

In some alternative embodiments, additional electrodes can be includedwithin the assembly, for example, a three-electrode system (working,reference, and counter electrodes) and/or an additional workingelectrode (e.g., an electrode which can be used to generate oxygen,which is configured as a baseline subtracting electrode, or which isconfigured for measuring additional analytes). U.S. Patent PublicationNo. 2005-0161346-A1, U.S. Patent Publication No. 2005-0143635-A1, andU.S. Patent Publication No. 2007-0027385-A1 describe some systems andmethods for implementing and using additional working, counter, and/orreference electrodes. In one implementation wherein the sensor comprisestwo working electrodes, the two working electrodes are juxtapositioned(e.g., extend parallel to each other), around which the referenceelectrode is disposed (e.g., helically wound). In some embodimentswherein two or more working electrodes are provided, the workingelectrodes can be formed in a double-, triple-, quad-, etc. helixconfiguration along the length of the sensor (for example, surrounding areference electrode, insulated rod, or other support structure). Theresulting electrode system can be configured with an appropriatemembrane system, wherein the first working electrode is configured tomeasure a first signal comprising glucose and baseline (e.g., backgroundnoise) signals and the additional working electrode is configured tomeasure a baseline signal only (e.g., configured to be substantiallysimilar to the first working electrode, but without an enzyme disposedthereon). In this way, the baseline signal can be subtracted from thefirst signal to produce a glucose-only signal that is substantially notsubject to fluctuations in the baseline and/or interfering species onthe signal.

In certain embodiments, the analyte sensor is configured as adual-electrode system and comprises a first working electrode and asecond working electrode, in addition to a reference electrode. Thefirst and second working electrodes may be in any useful conformation,as described in U.S. Patent Publication No. 2007-0027385-A1, U.S. PatentPublication No. 2007-0213611-A1, U.S. Patent Publication No.2007-0027284-A1, U.S. Patent Publication No. 2007-0032717-A1, U.S.Patent Publication No. 2007-0093704-A1, and U.S. Patent Publication No.2008-0083617-A1. In some embodiments, the first and second workingelectrodes are twisted and/or bundled. For example, two wire workingelectrodes can be twisted together, such as in a helix conformation. Thereference electrode can then be wrapped around the twisted pair ofworking electrodes. In some embodiments, the first and second workingelectrodes include a coaxial configuration. A variety of dual-electrodesystem configurations are described with reference to FIGS. 2G through211 of the references incorporated above. In some embodiments, thesensor is configured as a dual electrode sensor, such as described inU.S. Patent Publication No. 2005-0143635-A1, U.S. Patent Publication No.2007-0027385-A1, U.S. Patent Publication No. 2007-0213611-A1, and U.S.Patent Publication No. 2008-0083617-A1.

In certain embodiments, both of the working electrodes of adual-electrode analyte sensor are disposed beneath a sensor membrane,such as but not limited to a membrane system similar to that describedwith reference to FIGS. 6A-C, with the following exceptions. The firstworking electrode is disposed beneath an enzymatic enzyme domain (orportion of the sensor membrane) including an active enzyme configured todetect the analyte or an analyte-related compound. Accordingly, thefirst working electrode is configured to generate a first signalcomposed of both a signal related to the analyte and a signal related tonon-analyte electroactive compounds (e.g., physiological baseline,interferents, and non-constant noise) that have an oxidation/reductionpotential that overlaps with the oxidation/reduction potential of theanalyte. This oxidation/reduction potential may be referred to as a“first oxidation/reduction potential” herein. The second workingelectrode is disposed beneath a non-enzymatic enzyme domain (or portionof the sensor membrane) that includes either an inactivated form of theenzyme contained in the enzymatic portion of the membrane or no enzyme.In some embodiments, the non-enzymatic portion can include anon-specific protein, such as BSA, ovalbumin, milk protein, certainpolypeptides, and the like. The non-enzymatic portion generates a secondsignal associated with noise of the analyte sensor. The noise of thesensor comprises signal contribution due to non-analyte electroactivespecies (e.g., interferents) that have an oxidation/reduction potentialthat substantially overlaps the first oxidation/reduction potential(e.g., that overlap with the oxidation/reduction potential of theanalyte). In some embodiments of a dual-electrode analyte sensorconfigured for fluid communication with a host's circulatory system, thenon-analyte related electroactive species comprises at least one speciesselected from the group consisting of interfering species,non-reaction-related hydrogen peroxide, and other electroactive species.

In one exemplary embodiment, the dual-electrode analyte sensor is aglucose sensor having a first working electrode configured to generate afirst signal associated with both glucose and non-glucose relatedelectroactive compounds that have a first oxidation/reduction potential.Non-glucose related electroactive compounds can be any compound, in thesensor's local environment that has an oxidation/reduction potentialsubstantially overlapping with the oxidation/reduction potential ofH₂O₂, for example. While not wishing to be bound by theory, it isbelieved that the glucose-measuring electrode can measure both thesignal directly related to the reaction of glucose with GOx (producesH₂O₂ that is oxidized at the working electrode) and signals from unknowncompounds that are in the blood surrounding the sensor. These unknowncompounds can be constant or non-constant (e.g., intermittent ortransient) in concentration and/or effect. In some circumstances, it isbelieved that some of these unknown compounds are related to the host'sdisease state. For example, it is known that blood chemistry changesdramatically during/after a heart attack (e.g., pH changes, changes inthe concentration of various blood components/protein, and the like).Additionally, a variety of medicaments or infusion fluid components(e.g., acetaminophen, ascorbic acid, dopamine, ibuprofen, salicylicacid, tolbutamide, tetracycline, creatinine, uric acid, ephedrine,L-dopa, methyl dopa and tolazamide) that may be given to the host mayhave oxidation/reduction potentials that overlap with that of H₂O₂.

In this exemplary embodiment, the dual-electrode analyte sensor includesa second working electrode that is configured to generate a secondsignal associated with the non-glucose related electroactive compoundsthat have the same oxidation/reduction potential as the above-describedfirst working electrode. In some embodiments, the non-glucose relatedelectroactive species includes at least one of interfering species,non-reaction-related H₂O₂, and other electroactive species. For example,interfering species includes any compound that is not directly relatedto the electrochemical signal generated by the glucose-GOx reaction,such as but not limited to electroactive species in the localenvironment produces by other body processes (e.g., cellular metabolism,a disease process, and the like). Other electroactive species includesany compound that has an oxidation/reduction potential similar to oroverlapping that of H₂O₂.

The non-analyte (e.g., non-glucose) signal produced by compounds otherthan the analyte (e.g., glucose) may obscure the signal related to theanalyte, may contribute to sensor inaccuracy, and is consideredbackground noise. Background noise includes both constant andnon-constant components and is to be removed to accurately calculate theanalyte concentration. While not wishing to be bound by theory, it isbelieved that the sensor of some of the embodiments are designed (e.g.,with symmetry, coaxial design and/or integral formation, andinterference domain of the membrane described elsewhere herein) suchthat the first and second electrodes are influenced by substantially thesame external and/or environmental factors, which enables substantiallyequivalent measurement of both the constant and non-constantspecies/noise. This advantageously allows the substantial elimination ofnoise on the sensor signal (using electronics described elsewhereherein) to substantially reduce or eliminate signal effects due tonoise, including non-constant noise (e.g., unpredictable biological,biochemical species, medicaments, pH fluctuations, O₂ fluctuations, orthe like) known to effect the accuracy of conventional continuous sensorsignals. The sensor includes electronics may be operably connected tothe first and second working electrodes. The electronics are configuredto provide the first and second signals that are used to generateglucose concentration data substantially without signal contribution dueto non-glucose-related noise. The electronics can include at least apotentiostat that provides a bias to the electrodes. In someembodiments, sensor electronics are configured to measure the current(or voltage) to provide the first and second signals. The first andsecond signals are used to determine the glucose concentrationsubstantially without signal contribution due to non-glucose-relatednoise such as by but not limited to subtraction of the second signalfrom the first signal or alternative data analysis techniques. In someembodiments, the sensor electronics include a transmitter that transmitsthe first and second signals to a receiver, where additional dataanalysis and/or calibration of glucose concentration can be processed.U.S. Patent Publication No. 2005-0027463-A1, U.S. Patent Publication No.2005-0203360-A1, and U.S. Patent Publication No. 2006-0036142-A1describe systems and methods for processing sensor analyte data.

In some embodiments, the dual-electrode sensor is configured such thatthe first and second working electrodes are equivalently influenced byin vivo environmental factors. For example, in one embodiment, thedual-electrode sensor is configured for fluid communication with thecirculatory system of the host, such as by implantation in the host'svein or artery via a vascular access device (also referred to as a fluidcommunication device herein) such as a catheter and/or cannula. When thesensor is contacted with a sample of the host's circulatory system(e.g., blood), the first and second working electrodes are configuredsuch that they are equivalently influenced by a variety of environmentalfactors impinging upon the sensor, such as but not limited tonon-analyte related electroactive species (e.g., interfering species,non-reaction-related H₂O₂, another electroactive species). Because thefirst and second working electrodes are equivalently influenced by invivo environmental factors, the signal component associated with the invivo environmental factors (e.g., non-analyte related species with anoxidation/reduction potential that overlaps with that of the analyte)can be removed from the signal detected by the first working electrode(e.g., the first signal). This can give a substantially analyte-onlysignal.

In some embodiments, the surface area of the electroactive portion ofthe reference (and/or counter) electrode is at least six times thesurface area of the working electrodes. In other embodiments, thereference (and/or counter) electrode surface is at least 1, 2, 3, 4, 5,7, 8, 9 or 10 times the surface area of the working electrodes. In otherembodiments, the reference (and/or counter) electrode surface area is atleast 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 times the surface area ofthe working electrodes. For example, in a needle-type glucose sensor,similar to the embodiment shown in FIGS. 5A-5E, the surface area of thereference electrode (e.g., 114) includes the exposed surface of thereference electrode, such as but not limited to the electrode surfacefacing away from the working electrodes.

As a non-limiting example, in one embodiment, the dual-electrode analytesensor comprises a first working electrode configured to detect theanalyte and a second working electrode, wherein the first and secondworking electrodes are located on of two wire elongated conductivebodies E1, E2 twisted together to form a “twisted pair.” The firstworking electrode is disposed beneath an enzymatic portion of themembrane (not shown) containing an analyte-detecting enzyme. Forexample, in a glucose-detecting dual-electrode analyte sensor, aglucose-detecting enzyme, such as GOX, is included in the enzymaticportion of the membrane. Accordingly, the first working electrodedetects signal due to both the analyte and non-analyte-related speciesthat have an oxidation/reduction potential that substantially overlapswith the oxidation/reduction potential of the analyte. The secondworking electrode is disposed beneath a portion of the membranecomprising either inactivated enzyme (e.g., inactivated by heat,chemical or UV treatment) or no enzyme. Accordingly, the second workingelectrode detects a signal associated with only the non-analyteelectroactive species that have an oxidation/reduction potential thatsubstantially overlaps with that of analyte. For example, in theglucose-detecting dual-electrode analyte sensor described above, thenon-analyte (e.g., non-glucose) electroactive species have anoxidation/reduction potential that overlaps substantially with that ofH₂O₂. A reference electrode 114, such as a silver/silver chloride wireelectrode, is wrapped around the twisted pair. The three electrodes(e.g., working electrodes E1, E2 and the reference electrode 114) areconnected to sensor electronics (not shown), such as described elsewhereherein. In certain embodiments, the dual-electrode sensor is configuredto provide an analyte-only signal (e.g., glucose-only signal)substantially without a signal component due to the non-analyteelectroactive species (e.g., noise). For example, the dual-electrodesensor is operably connected to sensor electronics that process thefirst and second signals, such that a substantially analyte-only signalis provided (e.g., output to a user). In other exemplary embodiments,the dual-electrode sensor can be configured for detection of a varietyof analytes other than glucose, such as but not limited to urea,creatinine, succinate, glutamine, oxygen, electrolytes, cholesterol,lipids, triglycerides, hormones, liver enzymes, and the like.

In some embodiments, the analyte sensor substantially continuouslymeasures the host's analyte concentration. In some embodiments, forexample, the sensor can measure the analyte concentration every fractionof a second, about every fraction of a minute or every minute. In otherexemplary embodiments, the sensor measures the analyte concentration atleast about every 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes. In still otherembodiments, the sensor measures the analyte concentration everyfraction of an hour, such as but not limited to every 15, 30 or 45minutes. Yet in other embodiments, the sensor measures the analyteconcentration about every hour or longer. In some exemplary embodiments,the sensor measures the analyte concentration intermittently orperiodically. In one embodiment, the analyte sensor is a glucose sensorand measures the host's glucose concentration about every 4-6 minutes.In a further embodiment, the sensor measures the host's glucoseconcentration every 5 minutes.

As a non-limiting example, dual-electrode glucose sensor can bemanufactured as follows. In one embodiment, the conductive cores arefirst coated with a layer of insulating material (e.g., non-conductivematerial or dielectric) to prevent direct contact between conductivecores and the reference electrode 114. At this point, or at any pointhereafter, the two insulated conductive cores can be twisted and/orbundled to form a twisted pair. A portion of the insulator on anexterior surface of each conductive core is etched away, to expose theelectroactive surfaces of the working electrodes. In some embodiments,an enzyme solution (e.g., containing active GOx) is applied to theelectroactive surfaces of both working electrodes, and dried.Thereafter, the enzyme applied to one of the electroactive surfaces isinactivated. As is known in the art, enzymes can be inactivated by avariety of means, such as by heat, treatment with inactivating (e.g.,denaturing) solvents, proteolysis, laser irradiation or UV irradiation(e.g., at 254-320 nm). For example, the enzyme coating one of theelectroactive surfaces can be inactivated by masking one of theelectroactive surfaces/electrodes (e.g., temporarily covered with aUV-blocking material); irradiating the sensor with UV light (e.g.,254-320 nm; a wavelength that inactivates the enzyme, such as bycross-linking amino acid residues) and removing the mask. Accordingly,the GOx on the second working electrode is inactivated by the UVtreatment, but the first working electrode's GOx is still active due tothe protective mask. In other embodiments, an enzyme solution containingactive enzyme is applied to a first electroactive surface (e.g., firstworking electrode) and an enzyme solution containing either inactivatedenzyme or no enzyme is applied to the second electroactive surface(e.g., second working electrode). Thus, the enzyme-coated firstelectroactive surface detects analyte-related signal andnon-analyte-related signal, while the second electroactive surface,which lacks active enzyme, detects non-analyte-related signal. Asdescribed herein, the sensor electronics can use the data collected fromthe two working electrodes to calculate the analyte-only signal.

In some embodiments, the dual-electrode sensor system is configured forfluid communication with a host's circulatory system, such as via avascular access device. A variety of vascular access devices suitablefor use with a dual-electrode analyte sensor are described U.S. PatentPublication No. 2008-0119703-A1, U.S. Patent Publication No.2008-0108942-A1, U.S. Patent Publication No. 2008-0200789-A1.

FIG. 8A is a perspective view of the in vivo portion of anotherembodiment of a multi-electrode sensor system 800 comprising two workingelectrodes and at least one reference/counter electrode. The sensorsystem 800 comprises first and second elongated bodies E1, E2, eachformed of a conductive core or of a core with a conductive layerdeposited thereon. In this particular embodiment, an insulating layer810, a conductive layer 820, and a membrane layer (not shown) aredeposited on top of the elongated bodies E1, E2. The insulating layer810 separates the conductive layer 820 from the elongated body. Thematerials selected to form the insulating layer 810 may include any ofthe insulating materials described elsewhere herein, includingpolyurethane and polyimide. The materials selected to form theconductive layer 820 may include any of the conductive materialsdescribed elsewhere herein, including silver/silver chloride, platinum,gold, etc. Working electrodes 802′, 802″ are formed by removing portionsof the conductive layer 820 and the insulating layer 810, therebyexposing electroactive surface of the elongated bodies E1, E2,respectively. FIG. 8B provides a close perspective view of the distalportion of the elongated bodies E1, E2. FIG. 8C provides a front view ofthe sensor embodiment illustrated in FIGS. 8A and 8B.

The two elongated bodies illustrated in FIG. 8A are fabricated to havesubstantially the same shape and dimensions. In some embodiments, theworking electrodes are fabricated to have the same properties, therebyproviding a sensor system capable of providing redundancy of signalmeasurements. In other embodiments, the working electrodes, associatedwith the elongated bodies E1, E2, may each have one or morecharacteristics that distinguish each working electrode from the other.For example, in one embodiment, each of the elongated bodies E1, E2 maybe covered with a different membrane, so that each working electrode hasa different membrane property than the other working electrode. Forexample, one of the working electrodes may have a membrane comprising anenzyme layer and the other working electrode may have a membranecomprising a layer having either an inactivated form of the enzyme or noenzyme. Additional sensor system configurations that are possible with aplurality of working electrodes (e.g., sensor elements) are described inU.S. Patent Publication No. 2011-0024307-A1, which is incorporated byreference herein in its entirety.

Although not shown in FIGS. 8A-8C, in certain embodiments, the distalends 830′, 830″ of the core portions of the elongated bodies E1, E2 maybe covered with an insulating material (e.g., polyurethane orpolyimide). In alternative embodiments, the exposed core portions 830′,830″ may be covered with a membrane system and serve as additionalworking electrode surface area.

Regarding fabrication of the sensor system illustrated in FIG. 8A-8C, inone embodiment, two elongated bodies E1, E2 are provided. As describedabove, the elongated bodies E1, E2 may be formed as an elongatedconductive core, or alternatively as a core (conductive ornon-conductive) having at least one conductive material depositedthereon. Next, an insulating layer 810 is deposited onto each of theelongated bodies E1, E2. Thereafter, a conductive layer 820 is depositedover the insulating layer 810. The conductive layer 820 may serve as areference/counter electrode and may be formed of silver/silver chloride,or any other material that may be used for a reference electrode. Inalternative embodiments, the conductive layer 820 may be formed of adifferent conductive material, and may be used another workingelectrode. After these steps, a layer removal process is performed toremove portions of the deposited layers (i.e., the conductive layer 820and/or the insulating layer 810). Any of the techniques describedelsewhere herein (e.g., laser ablation, chemical etching, grit blasting)may be used. In the embodiment illustrated in FIGS. 8A and 8B, layers ofthe conductive layer 820 and the insulating layer 810 are removed toform the working electrodes 802′, 802″. Although in the embodimentshown, layer removal is performed across the entire cross-sectionalperimeter (e.g., circumference) of the deposited layer, it iscontemplated that in other embodiments, layer removal may be performedacross a preselected section of the cross-sectional perimeter, insteadof across the entire cross-sectional perimeter.

Contacts 804′, 804″ used to provide electrical connection between theworking electrodes and other components of the sensor system may beformed in a similar manner. As shown, contacts 804′ and 804″ areseparated from each other to prevent an electrical connectiontherebetween. Because the layer removal process is performed on eachindividual elongated body E1, E2, instead of a single geometricallycomplicated elongated body, this particular sensor design (i.e., twoelongated bodies placed side by side) may provide ease of manufacturing,as compared to the manufacturing processes involved with othermulti-electrode systems having other geometries.

After the conductive and insulating layers are deposited onto theelongated body, and after selected portions of the deposited layers havebeen removed, a membrane is applied onto at least a portion of theelongated bodies. In certain embodiments, the membrane system is appliedonly to the working electrodes, but in other embodiments the membranesystem is applied to the entire elongated body. In one embodiment, themembrane system is deposited onto the two working electrodessimultaneously while they are placed together (e.g., by bundling), butin another embodiment, membranes are deposited onto each individualworking electrode first, and the two working electrodes are then placedtogether.

FIG. 9A is a perspective view of the in vivo portion of anotherembodiment of a multi-electrode sensor system 900 comprising two workingelectrodes and one reference/counter electrode. The three electrodes areintegrated into one piece. The sensor system 900 comprises first,second, and third elongated bodies E1, E2, E3, each formed of aconductive core or of a core with a conductive layer deposited thereon.In this particular embodiment, an insulating domain 910 and a membranelayer (not shown) are deposited on top of an assembly comprising theelongated bodies E1, E2, E3. The insulating domain 910 binds the threeelongated bodies E1, E2, E3 in close proximity of each other, while alsoseparating them from direct contact with each other. The materialsselected to form the insulating domain 910 may include any of theinsulating materials described elsewhere herein, including polyurethaneand polyimide, for example. Working electrode 902′ on elongated body E1and another working electrode (not shown) on elongated body E2, areformed by removing portions of the insulating domain 910, therebyexposing electroactive surface of the elongated bodies E1, E2.Similarly, the reference electrode 904′ on elongated body E3 is alsoformed by removing portions of the insulating domain 910, therebyexposing electroactive surface of the elongated body E3. FIG. 9Bprovides a close perspective view of the distal portion of the elongatedbodies E1, E2, E3. FIG. 9C provides a front view of the sensorembodiment illustrated in FIGS. 9A and 9B.

As described elsewhere herein, in certain embodiments, the workingelectrodes, associated with the elongated bodies E1, E2, may each haveone or more characteristics that distinguish each working electrode fromthe other. For example, in some embodiments, one of the workingelectrodes may have a membrane comprising an enzyme layer and the otherworking electrode may have a membrane comprising a layer having eitheran inactivated form of the enzyme or no enzyme. Additional sensor systemconfigurations that are possible with a plurality of working electrodes(e.g., sensor elements) are described in U.S. Patent Publication No.2011-0024307-A1, which is incorporated by reference herein in itsentirety. In other embodiments, the working electrodes are fabricated tohave the same properties, thereby providing a sensor system capable ofproviding redundancy of signal measurements.

Although not shown in FIGS. 9A-9C, in certain embodiments, the distalends 930′, 930″, 930′″ of the core portions of the elongated bodies E1,E2, E3, respectively, may be covered with an insulating material (e.g.,polyurethane or polyimide). In alternative embodiments, one or more ofthe exposed core portions 930′, 930″, 930′″ may be covered with amembrane system and serve as additional working electrodes.

In one embodiment, fabrication of the sensor system illustrated in FIGS.9A-9C involves providing three elongated bodies E1, E2, E3. As describedabove, the elongated bodies may be formed as an elongated conductivecore, or alternatively as a core (conductive or non-conductive) havingat least one conductive material deposited thereon. The E3 referenceelectrode includes a core and/or an outer layer that comprises areference electrode material, such as, silver/silver chloride, forexample. Next, an insulating layer is deposited onto each of theelongated bodies. Thereafter, the three elongated bodies E1, E2, E3(with an insulating layer thereon) are placed together to form a singleelongated body. Although not required, in some embodiments, the threeelongated bodies E1, E2, E3 may be coated with a thermoplastic materialand fed through an aligning die. Afterwards, an insulating domain 910 isdeposited over this single elongated body. The deposited domain is thenallowed to dry or be cured, after which an unitary elongated body isformed, in which the three elongated bodies E1, E2, E3 are encased andheld together by insulating domain 910.

After these steps, a layer removal process is performed to removeportions of the insulating domain 910. Any of the techniques describedherein (e.g., laser ablation, chemical etching, grit blasting) may beused. In the embodiment illustrated in FIGS. 9A and 9B, portions of theinsulating domain 910 are removed to form the working electrode 902′ onelongated body E1, to form a second working electrode (not shown) onelongated body E2, and to form a reference electrode 904 on elongatedbody E3. Contacts 904′, 906 used to provide electrical connectionbetween the working and reference electrodes and other components of thesensor system may be formed in a similar manner.

FIG. 10A is a perspective view of the in vivo portion of yet anotherembodiment of a multi-electrode sensor system 1000 comprising twoworking electrodes and at least one reference/counter electrode. Thesensor system 1000 comprises first and second elongated bodies E1, E2,each formed of a conductive core or of a core with a conductive layerdeposited thereon. An insulating layer 1010 is deposited onto eachelongated body E1, E2. Furthermore, a conductive domain 1020 and amembrane layer (not shown) are deposited on top of an assemblycomprising the elongated bodies E1, E2 and the insulating layer. Theconductive domain 1020, binds the two elongated bodies E1, E2 into oneelongated body. The insulating layers 1010 surrounding each elongatedbody E1, E2 prevents electrical contact between the two elongated bodiesE1, E2. The materials selected to form the insulating layer 1010 mayinclude any of the insulating materials described elsewhere herein,including polyurethane and polyimide, for example. The materialsselected to form the conductive domain 1020 may include any of theconductive materials described elsewhere herein, including silver/silverchloride and platinum, for example. Working electrode 1002′ on elongatedbody E1 and another working electrode (not shown) on elongated body E2,are formed by removing portions of the conductive domain 1020 andportions of the insulating layer 1010, thereby exposing electroactivesurfaces of elongated bodies E1, E2. The portion of the conductivedomain 1020 not removed forms the reference/counter electrode. FIG. 10Bprovides a close perspective view of the distal portion of the elongatedbodies E1, E2. FIG. 10C provides a front view of the sensor embodimentillustrated in FIGS. 10A and 10B.

As described elsewhere herein, in certain embodiments, the workingelectrodes, associated with the elongated bodies E1, E2, may each haveone or more characteristics that distinguish each working electrode fromthe other. For example, in some embodiments, one of the workingelectrodes may have a membrane comprising an enzyme layer and the otherworking electrode may have a membrane comprising a layer having eitheran inactivated form of the enzyme or no enzyme. Additional sensor systemconfigurations that are possible with a plurality of working electrodes(e.g., sensor elements) are described in U.S. Patent Publication No.2011-0024307-A1, which is incorporated by reference herein in itsentirety. In other embodiments, the working electrodes are fabricated tohave the same properties, thereby providing a sensor system capable ofproviding redundancy of signal measurements.

Although not shown in FIGS. 10A-10C, in certain embodiments, the distalends 1030′, 1030″ of the core portions of the elongated bodies E1, E2,respectively, may be covered with an insulating material (e.g.,polyurethane or polyimide). In alternative embodiments, one or more ofthe exposed core portions 1030′, 1030″ may be covered with a membranesystem and serve as additional working electrodes.

In one embodiment, fabrication of the sensor system illustrated in FIGS.10A-10C involves providing two elongated bodies E1, E2. As describedabove, the elongated bodies may be formed as an elongated conductivecore, or alternatively as a core (conductive or non-conductive) havingat least one conductive material deposited thereon. Next, an insulatinglayer is deposited onto each of the elongated bodies. Thereafter, thetwo elongated bodies E1, E2 (with an insulating layer thereon) areplaced together to form a single elongated body. Although not required,in some embodiments, the three elongated bodies E1, E2, E3 may be coatedwith a thermoplastic material and fed through an aligning die.Afterwards, a conductive domain 1020 is deposited over this singleelongated body. The coated domain is then allowed to dry or be cured,after which the one unitary elongated body is formed, in which the twoelongated bodies E1, E2 are encased and held together by conductivedomain 1020.

After these steps, a layer removal process is performed to removeportions of the conductive domain 1020 and portions of the insulatinglayer 1010. Any of the techniques described herein (e.g., laserablation, chemical etching, grit blasting) may be used. In theembodiment illustrated in FIGS. 10A and 10B, portions of the conductivedomain 1020 and insulating layer 1010 are removed to form the workingelectrode 1002′ on elongated body E1, to form a second working electrode(not shown) on elongated body E2, and to form the reference electrode1020. Contacts 1004′, 1004″ used to provide electrical connectionbetween the working electrodes and other components of the sensor systemmay be formed in a similar manner.

With this particular sensor design, because the conductive domain 1020is disposed between the contact point between the two elongated bodiesE1, E2, the sensor system's largest cross-sectional dimension isminimized, as compared to a design in which both of the elongated bodieswere each individually covered with a conductive layer.

FIG. 11A is a perspective view of the in vivo portion of anotherembodiment of a multi-electrode sensor system 1100 comprising twoworking electrodes and one reference/counter electrode. The sensorsystem 1100 comprises first, second, and third elongated bodies E1, E2,E3, each formed of a conductive core or of a core with a conductivelayer deposited thereon. In this particular embodiment, an insulatinglayer 1110 and a membrane layer (not shown) are deposited on top of theelongated bodies E1, E2. The insulating layer 1110 separates theelongated bodies from each other. The materials selected to form theinsulating layer 1110 may include any of the insulating materialsdescribed elsewhere herein, including polyurethane and polyimide.Working electrodes 1102′, 1102″ are formed by removing portions of theinsulating layer 1110, thereby exposing electroactive surface of theelongated bodies E1, E2, respectively.

As described elsewhere herein, in certain embodiments, the workingelectrodes, associated with the elongated bodies E1, E2, may each haveone or more characteristics that distinguish each working electrode fromthe other. For example, in some embodiments, one of the workingelectrodes may have a membrane comprising an enzyme layer and the otherworking electrode may have a membrane comprising a layer having eitheran inactivated form of the enzyme or no enzyme. Additional sensor systemconfigurations that are possible with a plurality of working electrodes(e.g., sensor elements) are described in U.S. Patent Publication No.2011-0024307-A1, which is incorporated by reference herein in itsentirety. In other embodiments, the working electrodes are fabricated tohave the same properties, thereby providing a sensor system capable ofproviding redundancy of signal measurements.

Although not shown in FIGS. 11A-11C, in certain embodiments, the distalends 1130′, 1130″ of the core portions of the elongated bodies E1, E2may be covered with an insulating material (e.g., polyurethane orpolyimide). In alternative embodiments, the exposed core portions 1130′,1130″ may be covered with a membrane system and serve as additionalworking electrodes.

Regarding fabrication of the sensor system illustrated in FIG. 11A-11C,in one embodiment, three elongated bodies E1, E2, E3 are provided. Asdescribed above, the elongated bodies may be formed as an elongatedconductive core, or alternatively as a core (conductive ornon-conductive) having at least one conductive material depositedthereon. The elongated bodies E1, E2 that correspond to workingelectrodes may comprise an elongated core with a conductive materialtypically used with working electrodes (e.g., a core formed of aconductive material like platinum, or a core plated, coated, or claddedwith a conductive material like platinum). The elongated body E3 thatcorresponds to a reference electrode may comprise an elongated core witha conductive material typically used with reference electrodes (e.g., acore formed of a conductive material like silver/silver chloride, or acore plated, coated, or cladded with a conductive material likesilver/silver chloride).

Next, an insulating layer 1110 is deposited onto each of the elongatedbodies. In some embodiments, the insulating layer 1110 is formed of athermoplastic material, thereby allowing the three elongated bodies E1,E2, E3 to be attached together by a heating process that permits theinsulating layers of the three elongated bodies E1, E2, E3 to adheretogether.

Thereafter, a layer removal process is performed to remove portions ofthe insulating layer 1110. Any of the techniques described herein (e.g.,laser ablation, chemical etching, grit blasting) may be used. In theembodiment illustrated in FIGS. 11A and 11B, the insulating layer 1110is removed to form the working electrodes 1102′, 1102″ and referenceelectrode 1106. Contacts 1104′, 1104″ used to provide electricalconnection between the working electrodes and other components of thesensor system may be formed in a similar manner. As shown, contacts1104′ and 1104″ are separated from each other to prevent an electricalconnection therebetween. Because the layer removal process is performedon each individual elongated body, instead of a single geometricallycomplicated elongated body, this particular sensor design (i.e., twoelongated bodies placed side by side) may provide ease of manufacturing,as compared to the manufacturing involved with other sensors havingother geometries.

After the conductive and insulating layers have been deposited onto theelongated body, and after selected portions of the deposited layers havebeen removed, a membrane can be applied onto at least a portion of theelongated bodies. In certain embodiments, the membrane system is appliedonly to the working electrodes, but in other embodiments the membranesystem is applied to the entire elongated body. In one embodiment, amembrane system is deposited onto the two working electrodessimultaneously while they are placed together (e.g., by bundling), butin another embodiment, a membrane is deposited onto each individualworking electrode, and the two working electrodes are then placedtogether.

In one exemplary embodiment, the two elongated bodies E1, E2 are bundledtogether first (e.g., by providing adherence between the insulatinglayers of the working electrodes) to form a subassembly and an uncoatedsilver elongated conductive body E3 is then adhered to the subassemblyto form an assembly including all three elongated bodies E1, E2, E3.Subsequently, the silver elongated conductive body E3 can be chlorizedto form a silver/silver chloride reference electrode.

It should be understood that with any of the embodiments describedherein involving multiple working electrodes, one or more workingelectrodes may be designed to serve as an enzymatic electrode and one ormore working electrodes may be designed to serve as a “blank” workingelectrode configured to measure baseline. This configuration allows forsubtraction of a signal associated with the “blank” working electrode(i.e., the baseline non-analyte related signal) from the signalassociated with the enzymatic working electrode. The subtraction, inturn, results in a signal that contains substantially reduced (or no)non-analyte-related signal contribution (e.g., contribution frominterferents).

As described elsewhere herein, the elongated body may have any of avariety of cross-sectional shapes. This concept also applies tomulti-electrode sensors. For example, even though the elongated bodiesof the embodiment illustrated in FIGS. 10 A-10C is shown with a circularor substantially circular cross-sectional shape, it is contemplated thatother shapes may be used. By way of example, FIG. 10D is a perspectiveview of the in vivo portion of one embodiment having a sensor designsimilar to that of the embodiment illustrated in FIGS. 10A-10C, exceptthat that this particular embodiment has a substantially rectangularcross-section. The rectangular shape may provide advantages in certaininstances. For example, a rectangular shape design may provide a largerwindow area per unit length of etching (e.g., laser ablation). Thisresults in a larger electrode surface per unit of length of theelongated body, which in turn, allows for a sensor with a highersensitivity, as compared to an equivalent sensor with a circular crosssection. Additionally, the rectangular cross-section may allow foreasier handling (e.g., easier alignment) during the fabricationoperations, such as, extrusion, dip-coating, etching, and membraneapplications (e.g., with a “drop-on-demand” systems such asink-jetting.). Furthermore, a rectangular shape may provide for a morecompact cross-section, which allows for the sensor to be inserted with aneedle with a smaller diameter than an equivalent sensor with adifferent cross-sectional shape.

As described above, in some embodiments, a domain formed of a conductivematerial or an insulating material may be used to encase multipleelectrodes so that they are held together to form a unitary elongatedbody. In other embodiments, other types of material may be used insteadof (or in addition) to a conductive or insulating domain to holdtogether the multiple elongated bodies. FIG. 12 illustrates one suchembodiment. In this particular embodiment, the multi-electrode sensorsystem 1100 comprises two working electrodes associated with elongatedbodies E1, E2 and one reference/counter electrode associated withelongated body E3. Each of the elongated bodies E1, E2, E3 mayoptionally include an insulating layer 1110, portions of which areremoved to expose electroactive portions that correspond to working orreference electrodes. In addition, the elongated bodies E1, E2associated with the working electrodes include a membrane 1140. Althoughin this particular embodiment, the elongated body E3 associated with thereference electrode does not include a membrane, in alternativeembodiments, a membrane covers at least a portion of the referenceelectrode. A support structure 1150 is used to hold together themultiple elongated bodies E1, E2, E3. The support structure 1150 may bemade from any of a variety of materials, such as, polyurethane,silicone, or certain membrane materials. In addition, the supportstructure 1150 may contain a combination of hydrophobic and hydrophiliccomponents to ensure favorable bonding properties and permeability tothe underlying structure.

FIGS. 13A-13C illustrates another embodiment of a multi-electrode sensorsystem and the process for manufacturing it. As illustrated in FIG. 13A,an elongated body in the form of a carrier 1350 is provided whichincludes grooves 1360 that extend along the longitudinal axis of thecarrier 1350. The grooves 1360 are configured to hold a conductivepaste, such as, for example, a platinum paste to form a workingelectrode, or a silver/silver chloride paste to form a referenceelectrode. Although in this particular embodiment, the carrier 1350includes three grooves, in other embodiments, the carrier may includemore (e.g., 4, 5, 6, 7, 10, 15, 20) or less (e.g., 2) grooves to form asensor system with any number of working and reference electrodes. Incertain embodiments, the carrier 1350 is formed of a non-conductivematerial to prevent an electrical connection between the differentelectrodes. As shown in FIG. 13B, two of the grooves 1362, 1364 are thenfilled with a conductive material (e.g., a conductive metal paste suchas a platinum filled screen print ink) to form a working electrode, andone groove 1366 is filled with a different conductive material (e.g., asilver/silver chloride paste) to form a reference electrode. To fill thegrooves, a die can be used whereby channels are provided to guide pasteto the selected groove. To prevent cross-contamination of the materialsin between the grooves, a scraper can be provided at the exit end of thedie that scrapes conductive material to be flush with the carrier. Asillustrated in FIG. 13C, a membrane system 1310 can then applied to theassembly using any of the processes described elsewhere herein.

FIGS. 13D-13F illustrates another embodiment similar to the embodimentillustrated in FIGS. 13A-13C. As illustrated in FIG. 13D, in thisembodiment, a carrier 1350 is provided that includes two grooves 1368,1370. As shown in FIG. 13E, the two grooves 1368, 1370 are then filledwith a conductive material to form two working electrodes. Next, aninsulating layer 1310 is deposited onto the assembly, followed by thedeposition of a conductive layer 1320 which forms a reference electrode,as shown in FIGS. 13F-13G. Subsequently, the assembly undergoes anetching process (e.g., laser ablation, chemical etching, grit blasting)to remove certain portions of the insulating layer 1310 and conductivelayer 1320, thereby forming working electrodes.

Referring to FIG. 1A, in some embodiments, the sensor 100 comprises amembrane 108 in contact with the electroactive surface (e.g., at leastthe working electrode). As described elsewhere herein, an implantedsensor can be subjected to repeated bending and/or flexing along aplurality of axes. In order to withstand this harsh treatment and toprovide analyte data over 1, 2, 3 or more days, the sensor membrane isconfigured to withstand repeated compression, flexing and pulling whilesubstantially maintaining membrane function (e.g., without ripping,buckling or tearing). Accordingly, the sensor membrane may be strong yetelastic. Shore hardness is a measure of a material's elasticity. In someembodiments, the membrane comprises a polymer having a Shore hardness ofat least about 65 A, 70 A, 75 A, 80 A, 85 A, 90 A, 95 A, 30 D, 35 D, 40D, 45 D, 50 D, 55 D, 60 D, or more. In some embodiments, the polymer hasa Shore hardness of from about 70 A to about 55 D. Polymers having aShore hardness within the range of from about 70 A to about 55 D includebut are not limited to polyurethanes, polyimides, silicones, and thelike, such as described elsewhere herein.

In some embodiments, the entire membrane is formed of one or morepolymers having a Shore hardness of from about 70 A to about 55 D.However, in other embodiments, only a portion of the membrane, such as amembrane layer or domain, is formed of one of these polymers. In oneexemplary embodiment, an outer layer of the membrane is formed of apolymer having a Shore hardness of from about 70 A to about 55 D. Insome embodiments, approximately the outer 5%, 10%, 15%, 20%, 25%, 30%,35%, 35%, 40%, 45%, or 50% of the membrane is formed of a polymer havinga Shore hardness of from about 70 A to about 55 D. In some embodiments,the resistance domain is formed from a polymer having a Shore hardnessof from about 70 A to about 55 D. Additional membrane domains are alsoformed of a polymer having a Shore hardness within this range, in otherembodiments. In one embodiment, at least a portion of the membrane isformed of a polymer having a Shore hardness of from about 70 A to about55 D and an enzyme is disposed in the polymer.

As discussed herein, membranes can be formed by any suitable method. Itcan be desirable to form membranes on the various exposed electrodes ofthe sensors of some embodiments by dipping the sensor at differentlengths, by masking, controlled UV curing, and similar methods.

FIG. 6A is a cross section of the sensor shown in FIG. 1A, taken at line6-6. A membrane system (see FIG. 6A) is deposited over the electroactivesurfaces of the sensor and includes a plurality of domains or layers,such as described in more detail below. In some embodiments, themembrane comprises a single layer. In some embodiments, the single layerincludes one or more functional domains (e.g., portions or areas). Inother embodiments, the membrane comprises two or more layers. In someembodiments, each of the layers performs a different function.Alternative, multiple layers can perform the same function. The membranesystem can be deposited on the exposed electroactive surfaces usingknown thin film techniques (for example, spraying, electro-depositing,dipping, and the like). In one exemplary embodiment, each domain isdeposited by dipping the sensor into a solution and drawing out thesensor at a speed that provides the appropriate domain thickness. Inanother exemplary embodiment, each domain is deposited by spraying thesolution onto the sensor for a period of time that provides theappropriate domain thickness. In general, the membrane system can bedisposed over (deposited on) the electroactive surfaces using methodsappreciated by one skilled in the art.

In general, the membrane system includes a plurality of domains, forexample, an electrode domain 602, an interference domain 304, an enzymedomain 606 (for example, including glucose oxidase), and/or a resistancedomain 608, as shown in FIG. 6A, and can include a high oxygensolubility domain, and/or a bioprotective domain (not shown), such as isdescribed in more detail in U.S. Patent Publication No. 2005-0245799-A1,and such as is described in more detail below. The membrane system canbe deposited on the exposed electroactive surfaces using known thin filmtechniques (for example, vapor deposition, spraying, electro-depositing,dipping, and the like). In alternative embodiments, however, other vapordeposition processes (e.g., physical and/or chemical vapor depositionprocesses) can be useful for providing one or more of the insulatingand/or membrane layers, including ultrasonic vapor deposition,electrostatic deposition, evaporative deposition, deposition bysputtering, pulsed laser deposition, high velocity oxygen fueldeposition, thermal evaporator deposition, electron beam evaporatordeposition, deposition by reactive sputtering molecular beam epitaxy,atmospheric pressure chemical vapor deposition (CVD), atomic layer CVD,hot wire CVD, low-pressure CVD, microwave plasma-assisted CVD,plasma-enhanced CVD, rapid thermal CVD, remote plasma-enhanced CVD, andultra-high vacuum CVD, for example. However, the membrane system can bedisposed over (or deposited on) the electroactive surfaces using anyknown method, as will be appreciated by one skilled in the art. Whenenzymes are employed, e.g., in certain dual working electrode glucosesensor configurations, for ease of fabrication each of the electrodescan be dipped with an enzyme domain including enzyme. Enzyme over one ofthe working electrodes (e.g., the second working electrode) can then bedenatured/killed, e.g., by exposure to UV or other irradiation, or byexposure to other agents or treatment methods as known in the art fordenaturing enzyme. In some embodiments, one or more domains of themembrane systems are formed from materials such as silicone,polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene,homopolymers, copolymers, terpolymers of polyurethanes, polypropylene(PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF),polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA),polyether ether ketone (PEEK), polyurethanes, cellulosic polymers,polysulfones and block copolymers thereof including, for example,di-block, tri-block, alternating, random and graft copolymers. U.S.Patent Publication No. 2005-0245799-A1 describes biointerface andmembrane system configurations and materials that may be applied.

In selected embodiments, the membrane system comprises an electrodedomain. The electrode domain 602 is provided to ensure that anelectrochemical reaction occurs between the electroactive surfaces ofthe working electrode and the reference electrode, and thus theelectrode domain may be situated more proximal to the electroactivesurfaces than the interference and/or enzyme domain. The electrodedomain may include a coating that maintains a layer of water at theelectrochemically reactive surfaces of the sensor. In other words, theelectrode domain is present to provide an environment between thesurfaces of the working electrode and the reference electrode, whichfacilitates an electrochemical reaction between the electrodes. Forexample, a humectant in a binder material can be employed as anelectrode domain; this allows for the full transport of ions in theaqueous environment. The electrode domain can also assist in stabilizingthe operation of the sensor by accelerating electrode start-up anddrifting problems caused by inadequate electrolyte. The material thatforms the electrode domain can also provide an environment that protectsagainst pH-mediated damage that can result from the formation of a largepH gradient due to the electrochemical activity of the electrodes.

In one embodiment, the electrode domain includes hydrophilic polymerfilm (e.g., a flexible, water-swellable, hydrogel) having a “dry film”thickness of from about 0.05 microns or less to about 20 microns ormore, or from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45,0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or from about 3,2.5, 2, or 1 microns, or less, to about 3.5, 4, 4.5, or 5 microns ormore. “Dry film” thickness refers to the thickness of a cured film castfrom a coating formulation by standard coating techniques.

In certain embodiments, the electrode domain is formed of a curablemixture of a urethane polymer and a hydrophilic polymer. In someembodiments, the coatings are formed of a polyurethane polymer havingcarboxylate or hydroxyl functional groups and non-ionic hydrophilicpolyether segments, wherein the polyurethane polymer is crosslinked witha water-soluble carbodiimide (e.g.,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

In some embodiments, the electrode domain is formed from a hydrophilicpolymer (e.g., a polyamide, a polylactone, a polyimide, a polylactam, afunctionalized polyamide, a functionalized polylactone, a functionalizedpolyimide, a functionalized polylactam or a combination thereof) thatrenders the electrode domain substantially more hydrophilic than anoverlying domain, (e.g., interference domain, enzyme domain). In someembodiments, the electrode domain is formed substantially entirelyand/or primarily from a hydrophilic polymer. In some embodiments, theelectrode domain is formed substantially entirely from PVP. In someembodiments, the electrode domain is formed entirely from a hydrophilicpolymer. Useful hydrophilic polymers include but are not limited topoly-N-vinylpyrrolidone (PVP), poly-N-vinyl-2-piperidone,poly-N-vinyl-2-caprolactam, poly-N-vinyl-3-methyl-2-caprolactam,poly-N-vinyl-3-methyl-2-piperidone, poly-N-vinyl-4-methyl-2-piperidone,poly-N-vinyl-4-methyl-2-caprolactam, poly-N-vinyl-3-ethyl-2-pyrrolidone,poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, polyvinylimidazole,poly-N,N-dimethylacrylamide, polyvinyl alcohol, polyacrylic acid,polyethylene oxide, poly-2-ethyl-oxazoline, copolymers thereof andmixtures thereof. A blend of two or more hydrophilic polymers may beused in some embodiments. In some embodiments, the hydrophilicpolymer(s) is not crosslinked. In alternative embodiments, crosslinkingmay be performed, such as by adding a crosslinking agent, such as butnot limited to EDC, or by irradiation at a wavelength sufficient topromote crosslinking between the hydrophilic polymer molecules, which isbelieved to create a more tortuous diffusion path through the domain.

An electrode domain formed from a hydrophilic polymer (e.g., PVP) hasbeen shown to substantially reduce break-in time of analyte sensors; forexample, a glucose sensor utilizing a cellulosic-based interferencedomain such as described in more detail elsewhere herein. In someembodiments, a uni-component electrode domain formed from a singlehydrophilic polymer (e.g., PVP) has been shown to substantially reducebreak-in time of a glucose sensor to less than about 2 hours, less thanabout 1 hour, less than about 20 minutes and/or substantiallyimmediately, such as exemplified in Examples 9 through 11 and 13.Generally, sensor break-in is the amount of time required (afterimplantation) for the sensor signal to become substantiallyrepresentative of the analyte concentration. Sensor break-in includesboth membrane break-in and electrochemical break-in, which are describedin more detail elsewhere herein. In some embodiments, break-in time isless than about 2 hours. In other embodiments, break-in time is lessthan about 1 hour. In still other embodiments, break-in time is lessthan about 30 minutes, less than about 20 minutes, less than about 15minutes, less than about 10 minutes, or less. In one embodiment, sensorbreak-in occurs substantially immediately. Advantageously, inembodiments wherein the break-in time is about 0 minutes (substantiallyimmediately), the sensor can be inserted and begin providingsubstantially accurate analyte (e.g., glucose) concentrations almostimmediately post-insertion, for example, wherein membrane break-in doesnot limit start-up time.

While not wishing to be bound by theory, it is believed that providingan electrode domain that is substantially more hydrophilic than the nextmore distal membrane layer or domain (e.g., the overlaying domain; thelayer more distal to the electroactive surface than the electrodedomain, such as an interference domain or an enzyme domain) reduces thebreak-in time of an implanted sensor, by increasing the rate at whichthe membrane system is hydrated by the surrounding host tissue. Whilenot wishing to be bound by theory, it is believed that, in general,increasing the amount of hydrophilicity of the electrode domain relativeto the overlaying layer (e.g., the distal layer in contact withelectrode domain, such as the interference domain, enzyme domain, etc.),increases the rate of water absorption, resulting in reduced sensorbreak-in time. The hydrophilicity of the electrode domain can besubstantially increased by the proper selection of hydrophilic polymers,based on their hydrophilicity relative to each other and relative to theoverlaying layer (e.g., cellulosic-based interference domain), withcertain polymers being substantially more hydrophilic than theoverlaying layer. In one exemplary embodiment, PVP forms the electrodedomain, the interference domain is formed from a blend of cellulosicderivatives, such as but not limited to cellulose acetate butyrate andcellulose acetate; it is believed that since PVP is substantially morehydrophilic than the cellulosic-based interference domain, the PVPrapidly draws water into the membrane to the electrode domain, andenables the sensor to function with a desired sensitivity and accuracyand starting within a substantially reduced time period afterimplantation. Reductions in sensor break-in time reduce the amount oftime a host has to wait to obtain sensor readings, which is particularlyadvantageous not only in ambulatory applications, but particularly inhospital settings where time is critical.

While not wishing to be bound by theory, it is believed that when thewater absorption of the overlying domain (e.g., the domain overlying theelectrode domain) is less than the water absorption of the electrodedomain (e.g., during membrane equilibration), then the difference inwater absorption between the two domains will drive membraneequilibration and thus membrane break-in. Namely, increasing thedifference in hydrophilicity (e.g., between the two domains) results inan increase in the rate of water absorption, which, in turn, results ina decrease in membrane break-in time and/or sensor break-in time. Asdiscussed elsewhere herein, the relative hydrophilicity of the electrodedomain as compared to the overlying domain can be modulated by aselection of more hydrophilic materials for formation of the electrodedomain (and/or more hydrophobic materials for the overlying domain(s)).For example, an electrode domain with hydrophilic polymer capable ofabsorbing larger amounts of water can be selected instead of a secondhydrophilic polymer that is capable of absorbing less water than thefirst hydrophilic polymer. In some embodiments, the water contentdifference between the electrode domain and the overlying domain (e.g.,during or after membrane equilibration) is from about 1% or less toabout 90% or more. In other embodiments, the water content differencebetween the electrode domain and the overlying domain is from about 10%or less to about 80% or more. In still other embodiments, the watercontent difference between the electrode domain and the overlying domainis from about 30% or less to about 60% or more. In some embodiments, theelectrode domain absorbs 5 wt. % or less to 95 wt. % or more water, orfrom about 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 wt. % to about 55,60, 65, 70, 75, 80, 85, 90 or 95 wt. % water than the adjacent(overlying) domain (e.g., the domain that is more distal to theelectroactive surface than the electrode domain).

In another example, the rate of water absorption by a polymer can beaffected by other factors, such as but not limited to the polymer'smolecular weight. For example, the rate of water absorption by PVP isdependent upon its molecular weight, which is typically from about 40kDa or less to about 360 kDa or more; with a lower molecular weight PVP(e.g., 40 kDa) absorbing water faster than a higher molecular weightPVP. Accordingly, modulating factors, such as molecular weight, thataffect the rate of water absorption by a polymer, can promote the properselection of materials for electrode domain fabrication. In oneembodiment, a lower molecular weight PVP is selected, to reduce break-intime.

The electrode domain may be deposited by known thin film depositiontechniques (e.g., spray coating or dip-coating the electroactivesurfaces of the sensor). In some embodiments, the electrode domain isformed by dip-coating the electroactive surfaces in an electrode domainsolution (e.g., 5, 10, 15, 20, 25 or 30% or more PVP in deionized water)and curing the domain for a time of from about 15 minutes to about 30minutes at a temperature of from about 40° C. to about 55° C. (and canbe accomplished under vacuum (e.g., 20 to 30 mmHg)). In embodimentswherein dip-coating is used to deposit the electrode domain, aninsertion rate of from about 1 to about 3 inches per minute into theelectrode domain solution may be used, with a dwell time of from about0.5 to about 2 minutes in the electrode domain solution, and awithdrawal rate of from about 0.25 to about 2 inches per minute from theelectrode domain solution provide a functional coating. However, valuesoutside of those set forth above can be acceptable or even desirable incertain embodiments, for example, depending upon solution viscosity andsolution surface tension, as is appreciated by one skilled in the art.In one embodiment, the electroactive surfaces of the electrode systemare dip-coated one time (one layer) and cured at 50° C. under vacuum for20 minutes. In another embodiment, the electroactive surfaces of theelectrode system is dip-coated and cured at 50° C. under vacuum for 20minutes a first time, followed by dip coating and curing at 50° C. undervacuum for 20 minutes a second time (two layers). In still otherembodiments, the electroactive surfaces can be dip-coated three or moretimes (three or more layers). In other embodiments, the 1, 2, 3 or morelayers of PVP are applied to the electroactive surfaces by spray coatingor vapor deposition. In some embodiments, a crosslinking agent (e.g.,EDC) can be added to the electrode domain casting solution to promotecrosslinking within the domain (e.g., between electrode domain polymercomponents, latex, etc.). In some alternative embodiments however, nocrosslinking agent is used and the electrode domain is not substantiallycrosslinked.

In some embodiments, the deposited PVP electrode domain has a “dry film”thickness of from about 0.05 microns or less to about 20 microns, orfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. Although an independentelectrode domain is described herein, in some embodiments sufficienthydrophilicity can be provided in the interference domain and/or enzymedomain (the domain adjacent to the electroactive surfaces) so as toprovide for the full transport of ions in the aqueous environment (e.g.without a distinct electrode domain). In these embodiments, an electrodedomain is not necessary.

Interferents are molecules or other species that are reduced or oxidizedat the electrochemically reactive surfaces of the sensor, eitherdirectly or via an electron transfer agent, to produce a false positiveanalyte signal (e.g., a non-analyte-related signal). This false positivesignal causes the host's analyte concentration (e.g., glucoseconcentration) to appear higher than the true analyte concentration.False-positive signal is a clinically significant problem in someconventional sensors. For example in a case of a dangerouslyhypoglycemic situation, wherein the host has ingested an interferent(e.g., acetaminophen), the artificially high glucose signal can lead thehost to believe that he is euglycemic (or, in some cases,hyperglycemic). As a result, the host can make inappropriate treatmentdecisions, such as taking no action, when the proper course of action isto begin eating. In another example, in the case of a euglycemic orhyperglycemic situation, wherein a host has consumed acetaminophen, anartificially high glucose signal caused by the acetaminophen can leadthe host to believe that his glucose concentration is much higher thanit truly is. Again, as a result of the artificially high glucose signal,the host can make inappropriate treatment decisions, such as givinghimself too much insulin, which in turn can lead to a dangeroushypoglycemic episode.

In some embodiments, an interference domain 604 is provided thatsubstantially restricts or blocks the flow of one or more interferingspecies therethrough; thereby substantially preventing artificial signalincreases. Some known interfering species for a glucose sensor, asdescribed in more detail herein, include acetaminophen, ascorbic acid,bilirubin, cholesterol, creatinine, dopamine, ephedrine, ibuprofen,L-dopa, methyl dopa, salicylate, tetracycline, tolazamide, tolbutamide,triglycerides, and uric acid. In general, the interference domain ofsome embodiments is less permeable to one or more of the interferingspecies than to the measured species, e.g., the product of an enzymaticreaction that is measured at the electroactive surface(s), such as butnot limited to H₂O₂.

In one embodiment, the interference domain is formed from one or morecellulosic derivatives. Cellulosic derivatives can include, but are notlimited to, cellulose esters and cellulose ethers. In general,cellulosic derivatives include polymers such as cellulose acetate,cellulose acetate butyrate, 2-hydroxyethyl cellulose, cellulose acetatephthalate, cellulose acetate propionate, cellulose acetate trimellitate,and the like, as well as their copolymers and terpolymers with othercellulosic or non-cellulosic monomers. Cellulose is a polysaccharidepolymer of β-D-glucose. While cellulosic derivatives may be used in someembodiments, other polymeric polysaccharides having similar propertiesto cellulosic derivatives can also be employed in other embodiments.Descriptions of cellulosic interference domains can be found in U.S.Patent Publication No. 2006-0229512-A1, U.S. Patent Publication No.2007-0173709-A1, U.S. Patent Publication No. 2006-0253012-A1, and U.S.Patent Publication No. 2007-0213611-A1.

In some embodiments, the interferent's equivalent glucose signalresponse (measured by the sensor) is 50 mg/dL or less. In certainembodiments, the interferent produces an equivalent glucose signalresponse of 40 mg/dL or less. In some embodiments, the interferentproduces an equivalent glucose signal response of less than about 30, 20or 10 mg/dL. In one exemplary embodiment, the interference domain isconfigured to substantially block acetaminophen passage therethrough,wherein the equivalent glucose signal response of the acetaminophen isless than about 30 mg/dL.

In alternative embodiments, the interference domain is configured tosubstantially block a therapeutic dose of acetaminophen. The term“therapeutic dose” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to the quantity of any substance required toeffect the cure of a disease, to relieve pain, or that will correct themanifestations of a deficiency of a particular factor in the diet, suchas the effective dose used with therapeutically applied compounds, suchas drugs. For example, a therapeutic dose of acetaminophen can be anamount of acetaminophen required to relieve headache pain or reduce afever. As a further example, 1,000 mg of acetaminophen taken orally,such as by swallowing two 500 mg tablets of acetaminophen, is thetherapeutic dose frequently taken for headaches. In some embodiments,the interference membrane is configured to block a therapeutic dose ofacetaminophen, wherein the equivalent glucose signal response of theacetaminophen is less than about 60 mg/dL. In one embodiment, theinterference membrane is configured to block a therapeutic dose ofacetaminophen, wherein the equivalent glucose signal response of theacetaminophen is less than about 40 mg/dL. In another embodiment, theinterference membrane is configured to block a therapeutic dose ofacetaminophen, wherein the equivalent glucose signal response of theacetaminophen is less than about 30 mg/dL.

In some alternative embodiments, additional polymers, such as NAFION®,can be used in combination with cellulosic derivatives to provideequivalent and/or enhanced function of the interference domain. As oneexample, a layer of a 5 wt. % NAFION® casting solution was applied overa previously applied (e.g., and cured) layer of 8 wt. % celluloseacetate, e.g., by dip coating at least one layer of cellulose acetateand subsequently dip coating at least one layer NAFION® onto aneedle-type sensor. Any number of coatings or layers formed in any ordermay be suitable for forming the interference domain.

In some alternative embodiments, other polymer types that can beutilized as a base material for the interference domain includepolyurethanes, polymers having pendant ionic groups, and polymers havingcontrolled pore size, for example. In one such alternative embodiment,the interference domain includes a thin, hydrophobic membrane that isnon-swellable and restricts diffusion of high molecular weight species.The interference domain is permeable to relatively low molecular weightsubstances, such as hydrogen peroxide, but restricts the passage ofhigher molecular weight substances, including glucose and ascorbic acid.Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system are described in U.S.Pat. No. 7,074,307, U.S. Patent Publication No. 2005-0176136-A1, U.S.Pat. No. 7,081,195, and U.S. Patent Publication No. 2005-0143635-A1. Insome alternative embodiments, a distinct interference domain is notincluded.

In some embodiments, the interference domain is deposited eitherdirectly onto the electroactive surfaces of the sensor or onto thedistal surface of the electrode domain, for a domain thickness of fromabout 0.05 microns to about 20 microns, or from about 0.05, 0.1, 0.15,0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 micronsto about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or19.5 microns, or from about 1, 1.5 or 2 microns to about 2.5 or 3microns. Thicker membranes can also be desirable in certain embodiments,but thinner membranes may be used because they have a lower impact onthe rate of diffusion of hydrogen peroxide from the enzyme membrane tothe electroactive surface(s).

In general, the membrane systems of some embodiments can be formedand/or deposited on the exposed electroactive surfaces (e.g., one ormore of the working and reference electrodes) using known thin filmtechniques (for example, casting, spray coating, drawing down,electro-depositing, dip coating, and the like), however casting or otherknown application techniques can also be utilized. The interferencedomain may be deposited by spray or dip coating. In one exemplaryembodiment of a needle-type (transcutaneous) sensor such as describedherein, the interference domain is formed by dip coating the sensor intoan interference domain solution using an insertion rate of from about0.5 inch/min to about 60 inches/min, or about 1 inch/min, a dwell timeof from about 0 minute to about 2 minutes, or about 1 minute, and awithdrawal rate of from about 0.5 inch/minute to about 60 inches/minute,or about 1 inch/minute, and curing (drying) the domain from about 1minute to about 30 minutes, or from about 3 minutes to about 15 minutes(and can be accomplished at room temperature or under vacuum (e.g., 20to 30 mmHg)). In one exemplary embodiment including cellulose acetatebutyrate interference domain, a 3-minute cure (i.e., dry) time issometimes between each layer applied. In another exemplary embodimentemploying a cellulose acetate interference domain, a 15 minute cure(i.e., dry) time is used between each layer applied.

In some embodiments, the dip process can be repeated at least one timeand up to 10 times or more. In other embodiments, only one dip isperformed. The number of repeated dip processes used depends upon thecellulosic derivative(s) used, their concentration, conditions duringdeposition (e.g., dipping) and the desired thickness (e.g., sufficientthickness to provide functional blocking of certain interferents), andthe like. In some embodiments, 1 to 3 microns may be used for theinterference domain thickness; however, values outside of these can beacceptable or even desirable in certain embodiments, for example,depending upon viscosity and surface tension, as is appreciated by oneskilled in the art. In one exemplary embodiment, an interference domainis formed from three layers of cellulose acetate butyrate. In anotherexemplary embodiment, an interference domain is formed from 10 layers ofcellulose acetate. In another embodiment, an interference domain isformed from 1 layer of a blend of cellulose acetate and celluloseacetate butyrate. In alternative embodiments, the interference domaincan be formed using any known method and combination of celluloseacetate and cellulose acetate butyrate, as will be appreciated by oneskilled in the art. In some embodiments, the electroactive surface canbe cleaned, smoothed or otherwise treated prior to application of theinterference domain. In some embodiments, the interference domain ofsome embodiments can be useful as a bioprotective or biocompatibledomain, namely, a domain that interfaces with host tissue when implantedin an animal (e.g., a human) due to its stability and biocompatibility.In still other embodiments, other portions of the membrane system, suchas but not limited to the enzyme domain and/or the resistance domain canbe configured for interference blocking. In still other embodiments, aninterference domain can be located either more distally or proximally tothe electroactive surface than other membrane domains. For example, aninterference domain can be located more distal to the electroactivesurface than an enzyme domain or a resistance domain, in someembodiments.

In certain embodiments, the membrane system further includes an enzymedomain 606 disposed more distally from the electroactive surfaces thanthe interference domain; however other configurations can be desirable.In certain embodiments, the enzyme domain provides an enzyme to catalyzethe reaction of the analyte and its co-reactant, as described in moredetail below. In the some embodiments of a glucose sensor, the enzymedomain includes glucose oxidase (GOX); however other oxidases, forexample, galactose oxidase or uricase oxidase, can also be used. In someembodiments, the enzyme domain is configured and arranged for detectionof at least one of albumin, alkaline phosphatase, alanine transaminase,aspartate aminotransferase, bilirubin, blood urea nitrogen, calcium,CO₂, chloride, creatinine, glucose, gamma-glutamyl transpeptidase,hematocrit, lactate, lactate dehydrogenase, magnesium, oxygen, pH,phosphorus, potassium, sodium, total protein, uric acid, a metabolicmarker, a drug, various minerals, various metabolites, and/or the like.In a further embodiment, the sensor is configured and arranged to detecttwo or more of albumin, alkaline phosphatase, alanine transaminase,aspartate aminotransferase, bilirubin, blood urea nitrogen, calcium,CO₂, chloride, creatinine, glucose, gamma-glutamyl transpeptidase,hematocrit, lactate, lactate dehydrogenase, magnesium, oxygen, pH,phosphorus, potassium, sodium, total protein, uric acid, a metabolicmarker, a drug, various minerals, various metabolites, and/or the like.

For an enzyme-based electrochemical glucose sensor to perform well, thesensor's response may be limited by neither enzyme activity norco-reactant concentration. Because enzymes, including glucose oxidase,are subject to deactivation as a function of time even in ambientconditions, this behavior is compensated for in forming the enzymedomain. In some embodiments, the enzyme domain is constructed of aqueousdispersions of colloidal polyurethane polymers including the enzyme.However, in alternative embodiments the enzyme domain is constructedfrom an oxygen enhancing material, for example, silicone, orfluorocarbon, in order to provide a supply of excess oxygen duringtransient ischemia. The enzyme may be immobilized within the domain.See, e.g., U.S. Patent Publication No. 2005-0054909-A1. In someembodiments, the enzyme domain is deposited onto the interference domainfor a domain thickness of from about 0.05 micron to about 20 microns, orfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. However in some embodiments,the enzyme domain can be deposited directly onto the electroactivesurfaces. The enzyme domain may be deposited by spray or dip coating. Inone embodiment of needle-type (transcutaneous) sensor such as describedherein, the enzyme domain is formed by dip coating the interferencedomain coated sensor into an enzyme domain solution and curing thedomain for from about 15 to about 30 minutes at a temperature of fromabout 40° C. to about 55° C. (and can be accomplished under vacuum(e.g., 20 to 30 mmHg)). In embodiments wherein dip coating is used todeposit the enzyme domain at room temperature to provide a functionalcoating, the insertion rate used may be from about 0.25 inch per minuteto about 3 inches per minute, with a dwell time of from about 0.5minutes to about 2 minutes, and a withdrawal rate of from about 0.25inch per minute to about 2 inches per minute. However, values outside ofthose set forth above can be acceptable or even desirable in certainembodiments, for example, depending upon viscosity and surface tension,as is appreciated by one skilled in the art. In one embodiment, theenzyme domain is formed by dip coating two times (namely, forming twolayers) in an enzyme domain solution and curing at 50° C. under vacuumfor 20 minutes. However, in some embodiments, the enzyme domain can beformed by dip coating and/or spray coating one or more layers at apredetermined concentration of the coating solution, insertion rate,dwell time, withdrawal rate, and/or desired thickness.

Enzymatic analyte sensors are dependent upon the kinetics of the enzymesthat they comprise. As is understood by one skilled in the art, in orderto calculate the concentration of one reactant/analyte (e.g., using adefined amount of enzyme) all other reactants/co-reactants are presentin excess. However, in the body, this is often not the case; sometimesthe analyte is in excess. Accordingly, in some embodiments, the sensormembrane is configured and arranged to restrict diffusion of the analyteto the enzyme domain, such that the analyte can be measured accurately.In some embodiments, the membrane system includes a resistance domain608 disposed more distal from the electroactive surfaces than the enzymedomain. Although the following description is directed to a resistancedomain for a glucose sensor, the resistance domain can be modified forother analytes and co-reactants as well.

With respect to glucose sensors, there exists a molar excess of glucoserelative to the amount of oxygen in blood; that is, for every freeoxygen molecule in extracellular fluid, there are typically more than100 glucose molecules present (see Updike et al., Diabetes Care5:207-21(1982)). However, an immobilized enzyme-based glucose sensoremploying oxygen as co-reactant may be supplied with oxygen innon-rate-limiting excess in order for the sensor to respond linearly tochanges in glucose concentration, while not responding to changes inoxygen concentration. Specifically, when a glucose-monitoring reactionis oxygen limited, linearity is not achieved above minimalconcentrations of glucose. Without a semipermeable membrane situatedover the enzyme domain to control the flux of glucose and oxygen, alinear response to glucose levels can be obtained only for glucoseconcentrations of up to about 40 mg/dL. However, in a clinical setting,a linear response to glucose levels is desirable up to at least about400 mg/dL.

The resistance domain includes a semipermeable membrane that controlsthe flux of oxygen and glucose to the underlying enzyme domain, therebyrendering oxygen in a non-rate-limiting excess. As a result, the upperlimit of linearity of glucose measurement is extended to a much highervalue than that which is achieved without the resistance domain. In oneembodiment, the resistance domain exhibits an oxygen to glucosepermeability ratio of from about 50:1 or less to about 400:1 or more, orabout 200:1. As a result, one-dimensional reactant diffusion is adequateto provide excess oxygen at all reasonable glucose and oxygenconcentrations found in the subcutaneous matrix (See Rhodes et al.,Anal. Chem., 66:1520-1529 (1994)).

In alternative embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubilitydomain (for example, a silicone or fluorocarbon-based material ordomain) to enhance the supply/transport of oxygen to the enzyme domain.If more oxygen is supplied to the enzyme, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.In alternative embodiments, the resistance domain is formed from asilicone composition, such as is described in U.S. Patent PublicationNo. 2005-0090607-A1.

In one embodiment, the resistance domain includes a polyurethanemembrane with both hydrophilic and hydrophobic regions to control thediffusion of glucose and oxygen to an analyte sensor, the membrane beingfabricated easily and reproducibly from commercially availablematerials. A suitable hydrophobic polymer component is a polyurethane,or polyetherurethaneurea. Polyurethane is a polymer produced by thecondensation reaction of a diisocyanate and a difunctionalhydroxyl-containing material. A polyurethaneurea is a polymer producedby the condensation reaction of a diisocyanate and a difunctionalamine-containing material. Diisocyanates that may be used include, butare not limited to, aliphatic diisocyanates containing from about 4 toabout 8 methylene units. Diisocyanates containing cycloaliphaticmoieties can also be useful in the preparation of the polymer andcopolymer components of the membranes of some embodiments. The materialthat forms the basis of the hydrophobic matrix of the resistance domaincan be any of those known in the art as appropriate for use as membranesin sensor devices and as having sufficient permeability to allowrelevant compounds to pass through it, for example, to allow an oxygenmolecule to pass through the membrane from the sample under examinationin order to reach the active enzyme or electrochemical electrodes.Examples of materials which can be used to make non-polyurethane typemembranes include vinyl polymers, polyethers, polyesters, polyamides,inorganic polymers such as polysiloxanes and polycarbosiloxanes, naturalpolymers such as cellulosic and protein based materials, and mixtures orcombinations thereof.

In one embodiment, the hydrophilic polymer component is polyethyleneoxide. For example, one useful hydrophobic-hydrophilic copolymercomponent is a polyurethane polymer that includes about 20% hydrophilicpolyethylene oxide. The polyethylene oxide portions of the copolymer arethermodynamically driven to separate from the hydrophobic portions ofthe copolymer and the hydrophobic polymer component. The 20%polyethylene oxide-based soft segment portion of the copolymer used toform the final blend affects the water pick-up and subsequent glucosepermeability of the membrane.

In some embodiments, the resistance domain is formed from a siliconepolymer modified to allow analyte (e.g., glucose) transport.

In some embodiments, the resistance domain is formed from a siliconepolymer/hydrophobic-hydrophilic polymer blend. In one embodiment, Thehydrophobic-hydrophilic polymer for use in the blend may be any suitablehydrophobic-hydrophilic polymer, including but not limited to componentssuch as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. No. 4,803,243 and U.S. Pat. No. 4,686,044). In one embodiment, thehydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Suitable such polymers include,but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblockcopolymers, PEO-PPO-PEO triblock copolymers, alternating blockcopolymers of PEO-PPO, random copolymers of ethylene oxide and propyleneoxide, and blends thereof. In some embodiments, the copolymers may beoptionally substituted with hydroxy substituents. Commercially availableexamples of PEO and PPO copolymers include the PLURONIC® brand ofpolymers available from BASF®. In one embodiment, PLURONIC® F-127 isused. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers(e.g., PLURONIC® R products). Other suitable commercial polymersinclude, but are not limited to, SYNPERONICS® products available fromUNIQEMA®. U.S. Patent Publication No. 2007-0244379-A1 describes systemsand methods suitable for the resistance and/or other domains of themembrane system.

In some embodiments, the resistance domain is deposited onto the enzymedomain to yield a domain thickness of from about 0.05 microns to about20 microns, or from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4,0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, or from about2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. The resistancedomain may be deposited onto the enzyme domain by vapor deposition,spray coating, or dip coating. In one embodiment, spray coating is thepreferred deposition technique. The spraying process atomizes and miststhe solution, and therefore most or all of the solvent is evaporatedprior to the coating material settling on the underlying domain, therebyminimizing contact of the solvent with the enzyme.

In another embodiment, physical vapor deposition (e.g., ultrasonic vapordeposition) is used for coating one or more of the membrane domain(s)onto the electrodes, wherein the vapor deposition apparatus and processinclude an ultrasonic nozzle that produces a mist of micro-droplets in avacuum chamber. In these embodiments, the micro-droplets moveturbulently within the vacuum chamber, isotropically impacting andadhering to the surface of the substrate. Advantageously, vapordeposition as described above can be implemented to provide highproduction throughput of membrane deposition processes (e.g., at leastabout 20 to about 200 or more electrodes per chamber), greaterconsistency of the membrane on each sensor, and increased uniformity ofsensor performance, for example, as described below.

In some embodiments, depositing the resistance domain (for example,using one of the techniques described above) includes formation of amembrane system that substantially blocks or resists ascorbate (a knownelectrochemical interferent in hydrogen peroxide-measuring glucosesensors). While not wishing to be bound by theory, it is believed thatduring the process of depositing the resistance domain, a structuralmorphology is formed that is characterized in that ascorbate does notsubstantially permeate therethrough.

In one embodiment, the resistance domain is deposited on the enzymedomain by spray coating a solution of from about 1 wt. % to about 5 wt.% polymer and from about 95 wt. % to about 99 wt. % solvent. In sprayinga solution of resistance domain material, including a solvent, onto theenzyme domain, it is desirable to mitigate or substantially reduce anycontact with enzyme of any solvent in the spray solution that candeactivate the underlying enzyme of the enzyme domain. Tetrahydrofuran(THF) is one solvent that minimally or negligibly affects the enzyme ofthe enzyme domain upon spraying. Other solvents can also be suitable foruse, as is appreciated by one skilled in the art.

Although a variety of spraying or deposition techniques can be used,spraying the resistance domain material and rotating the sensor at leastone time by 180° can typically provide adequate coverage by theresistance domain. Spraying the resistance domain material and rotatingthe sensor at least two times by 120° provides even greater coverage(one layer of 360° coverage), thereby ensuring resistivity to glucose,such as is described in more detail above.

In some embodiment, the resistance domain is spray coated andsubsequently cured for a time of from about 15 minutes to about 90minutes at a temperature of from about 40° C. to about 60° C. (and canbe accomplished under vacuum (e.g., from 20 to 30 mmHg)). A cure time ofup to about 90 minutes or more can be advantageous to ensure completedrying of the resistance domain.

In one embodiment, the resistance domain is formed by spray coating atleast six layers (namely, rotating the sensor seventeen times by 120°for at least six layers of 360° coverage) and curing at 50° C. undervacuum for 60 minutes. However, the resistance domain can be formed bydip coating or spray coating any layer or plurality of layers, dependingupon the concentration of the solution, insertion rate, dwell time,withdrawal rate, and/or the desired thickness of the resulting film.Additionally, curing in a convention oven can also be employed.

In certain embodiments, a variable frequency microwave oven can be usedto cure the membrane domains/layers. In general, microwave ovensdirectly excite the rotational mode of solvents. Consequently, microwaveovens cure coatings from the inside out rather than from the outside inas with conventional convection ovens. This direct rotational modeexcitation is responsible for the typically observed “fast” curingwithin a microwave oven. In contrast to conventional microwave ovens,which rely upon a fixed frequency of emission that can cause arcing ofdielectric (metallic) substrates if placed within a conventionalmicrowave oven, Variable Frequency Microwave (VFM) ovens emit thousandsof frequencies within 100 milliseconds, which substantially eliminatesarcing of dielectric substrates. Consequently, the membranedomains/layers can be cured even after deposition on metallic electrodesas described herein. While not wishing to be bound by theory, it isbelieve that VFM curing can increase the rate and completeness ofsolvent evaporation from a liquid membrane solution applied to a sensor,as compared to the rate and completeness of solvent evaporation observedfor curing in conventional convection ovens. In certain embodiments, VFMis can be used together with convection oven curing to furtheraccelerate cure time. In some sensor applications wherein the membraneis cured prior to application on the electrode (see, for example, U.S.Patent Publication No. 2005-0245799-A1, conventional microwave ovens(e.g., fixed frequency microwave ovens) can be used to cure the membranelayer.

A variety of therapeutic (bioactive) agents can be used with the analytesensor system of some embodiments. In some embodiments, the therapeuticagent is an anticoagulant. In some embodiments, an anticoagulant isincluded in the analyte sensor system to prevent coagulation within oron the sensor (e.g., within or on the catheter or within or on thesensor). In some embodiments, the therapeutic agent is an antimicrobial,such as but not limited to an antibiotic or antifungal compound. In someembodiments, the therapeutic agent is an antiseptic and/or disinfectant.Therapeutic agents can be used alone or in combination of two or more ofthem. The therapeutic agents can be dispersed throughout the material ofthe sensor (and/or catheter). In some embodiments, the membrane systemof some embodiments includes a therapeutic agent, which is incorporatedinto at least a portion of the membrane system, or which is incorporatedinto the device and adapted to diffuse through the membrane. There are avariety of systems and methods by which the therapeutic agent isincorporated into the membrane. In some embodiments, the therapeuticagent is incorporated at the time of manufacture of the membrane system.For example, the therapeutic agent can be blended prior to curing themembrane system, or subsequent to membrane system manufacture, forexample, by coating, imbibing, solvent-casting, or sorption of thebioactive agent into the membrane system. Although the therapeutic agentmay be incorporated into the membrane system, in some embodiments thetherapeutic agent can be administered concurrently with, prior to, orafter insertion of the device intravascularly, for example, by oraladministration, or locally, for example, by subcutaneous injection nearthe implantation site. A combination of therapeutic agent incorporatedin the membrane system and therapeutic agent administration locallyand/or systemically can be used in certain embodiments.

As a non-limiting example, in some embodiments, the analyte sensor 100is a continuous electrochemical analyte sensor configured to provide atleast one working electrode and at least one reference electrode, whichare configured to measure a signal associated with a concentration ofthe analyte in the host, such as described in more detail below. Theoutput signal is typically a raw data stream that is used to provide auseful value of the measured analyte concentration in a host to thepatient or doctor, for example. However, the analyte sensors of someembodiments comprise at least one additional working electrodeconfigured to measure at least one additional signal, as discussedelsewhere herein. For example, in some embodiments, the additionalsignal is associated with the baseline and/or sensitivity of the analytesensor, thereby enabling monitoring of baseline and/or sensitivitychanges that may occur over time.

In general, electrochemical continuous analyte sensors define arelationship between sensor-generated measurements (for example, currentin pA, nA, or digital counts after A/D conversion) and a referencemeasurement (for example, glucose concentration mg/dL or mmol/L) thatare meaningful to a user (for example, patient or doctor). For example,in the case of an implantable diffusion-based glucose oxidaseelectrochemical glucose sensor, the sensing mechanism generally dependson phenomena that are linear with glucose concentration, for example:(1) diffusion of glucose through a membrane system (for example,biointerface membrane and membrane system) situated between implantationsite and/or the electrode surface, (2) an enzymatic reaction within themembrane system, and (3) diffusion of the H₂O₂ to the sensor. Because ofthis linearity, calibration of the sensor can be understood by solvingan equation:

y=mx+b

wherein y represents the sensor signal (e.g., counts), x represents theestimated glucose concentration (e.g., mg/dL), m represents the sensorsensitivity to glucose (e.g., counts/mg/dL), and b represents thebaseline signal (e.g., counts). When both sensitivity m and baseline(background) b change over time in vivo, calibration has generallyrequires at least two independent, matched data pairs (x₁, y₁; x₂, y₂)to solve for m and b and thus allow glucose estimation when only thesensor signal, y is available. Matched data pairs can be created bymatching reference data (for example, one or more reference glucose datapoints from a blood glucose meter, or the like) with substantially timecorresponding sensor data (for example, one or more glucose sensor datapoints) to provide one or more matched data pairs, such as described inco-pending U.S. Patent Publication No. 2005-0027463-A1. In someimplantable glucose sensors, such as described in more detail in U.S.Pat. No. 6,329,161 to Heller et al., the sensing layer utilizesimmobilized mediators (e.g., redox compounds) to electrically connectthe enzyme to the working electrode, rather than using a diffusionalmediator. In some implantable glucose sensors, such as described in moredetail in U.S. Pat. No. 4,703,756, the system has two oxygen sensorssituated in an oxygen-permeable housing, one sensor being unaltered andthe other contacting glucose oxidase allowing for differentialmeasurement of oxygen content in body fluids or tissues indicative ofglucose levels. A variety of systems and methods of measuring glucose ina host are known, all of which may benefit from some of all of theembodiments to provide a sensor having a signal-to-noise ratio that isnot substantially affected by non-constant noise.

Advantageously, continuous analyte monitoring is enabled. For example,when the analyte is glucose, continuous glucose monitoring enables tightglucose control, which can lead to reduced morbidity and mortality amongdiabetic hosts. In some embodiments, the medical staff is not undulyburdened by additional patient interaction requirements. Advantageously,there is no net sample (e.g., blood) loss for the host, which is acritical feature in some clinical settings. For example, in a neonatalintensive care unit, the host is extremely small and loss of even a fewmilliliters of blood can be life threatening. Furthermore, returning thebody fluid sample to the host, instead of delivering to a wastecontainer greatly reduces the accumulation of biohazardous waste thatrequires special disposal procedures. The integrated sensor systemcomponents, as well as their use in conjunction with an indwellinganalyte sensor, are discussed in greater detail below.

A variety of known sensor configurations can be employed with the sensorsystems described herein, such as U.S. Pat. No. 5,711,861 to Ward etal., U.S. Pat. No. 6,642,015 to Vachon et al., U.S. Pat. No. 6,654,625to Say et al., U.S. Pat. No. 6,565,509 to Say et al., U.S. Pat. No.6,514,718 to Heller, U.S. Pat. No. 6,465,066 to Essenpreis et al., U.S.Pat. No. 6,214,185 to Offenbacher et al., U.S. Pat. No. 5,310,469 toCunningham et al., and U.S. Pat. No. 5,683,562 to Shaffer et al., U.S.Pat. No. 6,579,690 to Bonnecaze et al., U.S. Pat. No. 6,484,046 to Sayet al., U.S. Pat. No. 6,512,939 to Colvin et al., U.S. Pat. No.6,424,847 to Mastrototaro et al., U.S. Pat. No. 6,424,847 toMastrototaro et al, U.S. Patent Publication No. 2006-0020187-A1 toBrister et al., U.S. Patent Publication No. 2007-0027370-A1 to Braukeret al., U.S. Patent Publication No. 2005-0143635-A1 to Kamath et al.,U.S. Patent Publication No. 2007-0027385-A1 to Brister et al., U.S.Patent Publication No. 2007-0213611-A1 to Simpson et al., U.S. PatentPublication No. 2008-0083617-A1 to Simpson et al., U.S. PatentPublication No. 2008-0119703-A1 to Brister et al., U.S. PatentPublication No. 2008-0108942-A1 to Brister et al., and U.S. PatentPublication No. 2009-0018424-A1 Kamath et al., for example. Theabove-referenced patents and publications are not inclusive of allapplicable analyte sensors; in general, it should be understood that thedisclosed embodiments are applicable to a variety of analyte sensorconfigurations.

In some embodiments, the dual-electrode sensor includes electronics(e.g., a processor module, processing memory) that are operablyconnected to the first and second working electrodes and are configuredto provide the first and second signals to generate analyteconcentration data substantially without signal contribution due tonon-analyte-related noise. For example, the sensor electronics processand/or analyze the signals from the first and second working electrodesand calculate the portion of the first electrode signal that is due toanalyte concentration only. The portion of the first electrode signalthat is not due to the analyte concentration can be considered to bebackground, such as but not limited to noise. Accordingly, in oneembodiment of a dual-electrode sensor system configured for fluidcommunication with a host's circulatory system (e.g., via a vascularaccess device) the system comprising electronics operably connected tothe first and second working electrodes; the electronics are configuredto process the first and second signals to generate analyteconcentration data substantially without signal contribution due tonoise.

In certain embodiments, the sensor electronics (e.g., electroniccomponents) are operably connected to the first and second workingelectrodes. The electronics are configured to calculate at least oneanalyte sensor data point. For example, the electronics can include apotentiostat, A/D converter, RAM, ROM, transmitter, and the like. Insome embodiments, the potentiostat converts the raw data (e.g., rawcounts) collected from the sensor to a value familiar to the host and/ormedical personnel. For example, the raw counts from a glucose sensor canbe converted to milligrams of glucose per deciliter of glucose (e.g.,mg/dL). In some embodiments, the electronics are operably connected tothe first and second working electrodes and are configured to processthe first and second signals to generate a glucose concentrationsubstantially without signal contribution due to non-glucose noiseartifacts. The sensor electronics determine the signals from glucose andnon-glucose related signal with an overlapping measuring potential(e.g., from a first working electrode) and then non-glucose relatedsignal with an overlapping measuring potential (e.g., from a secondelectrode). The sensor electronics then use these data to determine asubstantially glucose-only concentration, such as but not limited tosubtracting the second electrode's signal from the first electrode'ssignal, to give a signal (e.g., data) representative of substantiallyglucose-only concentration, for example. In general, the sensorelectronics may perform additional operations, such as but not limitedto data smoothing and noise analysis.

In certain embodiments, the dual-electrode sensor includes electronics(e.g., a processor module, processing memory) that are operablyconnected to the first and second working electrodes and are configuredto provide the first and second signals to generate an analyteconcentration data substantially without signal contribution due tonon-analyte-related noise. For example, the sensor electronics processand/or analyze the signals from the first and second working electrodesand calculate the portion of the first electrode signal that is due toanalyte concentration only. The portion of the first electrode signalthat is not due to the analyte concentration can be considered to bebackground, such as but not limited to noise. Accordingly, in oneembodiment of a dual-electrode sensor system configured for fluidcommunication with a host's circulatory system (e.g., via a vascularaccess device) the system comprising electronics operably connected tothe first and second working electrodes; the electronics are configuredto process the first and second signals to generate analyteconcentration data substantially without signal contribution due tonoise.

In some embodiments, the dual-electrode analyte sensor includes areference sensor/system, as described elsewhere therein, wherebyreference data can be provided for calibration (e.g., internal to thesystem), without the use of an external (e.g., separate from the system)analyte-measuring device. In an exemplary embodiment, the dual-electrodesensor is a glucose sensor and external glucose data points (e.g., froma hand-held glucose meter or a YSI device) are not required forcalibration of a dual-electrode glucose sensor system that includes areference sensor. In some embodiments, the reference sensor isconfigured to be disposed within the same local environment as thedual-electrode analyte sensor, such that the reference sensor and thedual-electrode analyte sensor can be simultaneously exposed to a sample.In some embodiments, the reference sensor/system can be disposedremotely from the dual-electrode sensor. In these embodiments, theelectronics module is configured to process the reference data with thefirst and second signals to generate analyte concentration datasubstantially without signal contribution due to noise. In someembodiments, the electronics module is configured to calibrate thedual-electrode analyte sensor data using the reference sensor data, asdescribed elsewhere herein.

In some embodiments, the electronics module is configured to determine ascaling factor (k) as described in the section entitled “CalibrationSystems and Methods.” Briefly, a scaling factor defines a relationshipbetween the enzymatic portion of the membrane and the non-enzymaticportion of the membrane. Accordingly, in some embodiments, theelectronics module, also referred to as the processor module herein, isconfigured to calibrate the analyte sensor data using the scalingfactor, such that the calibrated sensor data does not includeinaccuracies that can arise due to small differences between the plus-and minus-enzyme portions of the membrane at the first and secondworking electrodes, respectively.

In some embodiments, the system is configured to calibrate thecontinuous dual-electrode analyte sensor using a reference fluid (e.g.,602 a), as described in the section entitled “integrated sensor system.”In some embodiments, the system is configured to calibrate the sensorusing single-point calibration, in other embodiments, the system isconfigured to calibrate the sensor without a reference data pointprovided by an external analyte monitor (e.g., SMBG, YSI), as describedelsewhere herein. In some embodiments, the system includes a referencesensor configured to generate a signal associated with a referenceanalyte in the sample (e.g., internal to the system), wherein thecontinuous analyte sensor is further configured to generate a thirdsignal associated with the reference analyte, and wherein the system isconfigured to calibrate the continuous analyte sensor using thereference signal and the third signal. In some embodiments, thereference sensor comprises an optical sensing apparatus, such as but notlimited to an optical O₂ sensor. In some embodiments, the continuousanalyte sensor is a glucose sensor. In other embodiments, a substantialportion of the continuous analyte sensor has a diameter of less thanabout 0.008 inches, as is described elsewhere herein.

In some further embodiments, the continuous analyte sensor furthercomprises a bioinert material or a bioactive agent incorporated thereinor thereon. Applicable bioactive agent include but are not limited tovitamin K antagonists, heparin group anticoagulants, plateletaggregation inhibitors, enzymes, direct thrombin inhibitors, Dabigatran,Defibrotide, Dermatan sulfate, Fondaparinux, and Rivaroxaban.

As a non-limiting example, in some embodiments, a method forcontinuously detecting an analyte in the host in vivo using adual-electrode analyte sensor is provided. In some embodiments, avascular access device (e.g., a catheter) is inserted into the host'scirculatory system, such as into a vein or artery. The sensor iscontacted with a sample of the circulatory system, such as a sample ofblood withdrawn into the catheter. A first signal is generated by thesensor, wherein the first signal is associated with associated with theanalyte and non-analyte related electroactive compounds having a firstoxidation/reduction potential in a sample of the circulatory system ofthe host. In certain embodiments, the analyte sensor is configured todetect glucose. A second signal is also generated, wherein the secondsignal is associated with noise of the analyte sensor, wherein the noisecomprises signal contribution due to non-analyte related electroactivespecies with an oxidation/reduction potential that substantiallyoverlaps with the first oxidation/reduction potential in the sample. Thefirst and second signals are processed to provide a processed signalsubstantially without a signal component associated with noise. In someembodiments, the first and second signals are processed to provide ascaling factor, which can then be used to calibrate the first signal. Insome embodiments, a reference sensor is also contacted with the sample,and a third signal associated with a reference analyte generated. Insome embodiments, the reference sensor is an optical detectionapparatus, such as but not limited to an optical O₂ sensor. In thisembodiment, the first and second signals can be calibrated using thethird and/or reference signal. In some embodiments, the processing stepcomprises evaluating steady-state information and transient information,wherein the first and second signals each comprise steady-state andtransient information. In some further embodiments, the evaluating stepincludes evaluating at least one of sensitivity information and baselineinformation, wherein the steady-state information comprises thesensitivity and baseline information.

Example

Test sensors were designed and fabricated with selection of certainmaterials, processing, and structure that improve fatigue life, and yetalso improve comfort for the user. During the fabrication process, anelongated conductive body is first built by inserting/slip fitting a rodor wire into a tube formed of a conductive material, the combination ofwhich forms an initial structure of an elongated conductive body. Theresulting elongated conductive body is then passed through a series ofdies to draw down the diameter of the elongated conductive body from alarge diameter to a small diameter. With each pass through the die, thecross-sectional profile of the elongated conductive body is compressed,and the diameter associated therewith is reduced. Between passes throughthe die, an annealing step is performed to cause changes in themechanical and structural properties of the elongated conductive body,and to relieve internal stresses, refine the structure by making it morehomogeneous, and improve general cold working properties. It has beenfound that drawing down the diameter of the elongated conductive bodythrough large numbers of dies in small incremental steps, instead ofthrough one or a few number of large incremental step(s), can result incertain mechanical and structural properties that improve fatigue life.It has also been found that performing the annealing on the elongatedconductive body in between drawing passes also improves fatigue life andprovided additional comfort for the user. Afterwards, a polyurethane anda silver/silver chloride layer are coated onto the elongated conductivebody, followed by deposition of a membrane (with an electrode, enzyme,and resistance layer).

The fabricated test sensors were then tested with a fatigue measurementdevice 1410 to determine test sensor fatigue lives under conditions thatbetter models subcutaneous conditions than other conventional models. Asillustrated in FIG. 14, the fatigue life measurement device 1410includes clamp members 1420, holding members 1440, and fixtures 1430used to hold and support the test sensor 1450. The fatigue lifemeasurement device 1410 also includes a rotator 1460 that rotates about45 degrees in each direction for a total of about 90 degrees per cycle.The radius of the holding members 1440 are about 0.031 inches. Duringtesting, the rotator 1460 rotates back and forth at a constant rate,thereby causing the sensor 1450 to bend back and forth at a degreecorresponding to the radius of the holding members 1440.

FIG. 15 illustrates a table summarizing the results of the performanceof test sensors with conventional sensors, with respect to fatigue life.As shown, the test sensors using the fabrication techniques describedabove were able to achieve substantially longer fatigue lives thanconventional sensors. Indeed the average fatigue life of the testsensors was about 61 cycles, as compared to the average fatigue life ofthe conventional sensors which was about 13.87 cycles. Accordingly, thefabrication methods described herein were able to extend sensor fatiguelife by a factor over four. Indeed, sensors built in accordance with theembodiments described herein can achieve a fatigue life of greater than20 cycles, greater than 40 cycles, greater than 60 cycles, or greaterthan 65 cycles.

Electrode Regeneration

Devices, systems, and methods for increasing reference electrodecapacity of an analyte sensor are provided. A typical analyte sensor caninclude a working electrode and a reference electrode, and optionallyone or more additional working electrodes. Silver/silver chloridereference electrodes have been used in continuous analyte sensors, suchas transcutaneous and subcutaneous electrochemical glucose sensors.However, due to significant depletion of silver chloride during typicalperiods of usage, e.g., days, weeks, or more, this reduction inelectrode capacity can substantially limit the effectiveness of thereference electrode over time. During use, the silver-chloride componentof the reference electrode is reduced to silver and chloride accordingto the following reaction:

AgCl+e−→Ag+Cl—

When the silver chloride becomes completely or substantially depleted,the reference electrode potential loses stability, such that the sensorresponse to the analyte becomes non-linear. For proper functioning ofthe sensor, however, the reference electrode potential should besubstantially stable for the duration of the sensor life at (or above)the plateau potential for the analyte. An advantage of certain systemsand methods of various embodiments is the ability to increase thereference capacity of the reference electrode by preventing prematuredepletion of silver chloride, such that the useful life of the sensor isextended.

It has been found that in certain embodiments, the reference capacity ofthe reference electrode can be increased, if at least a portion of thereference electrode is covered with an enzyme layer. The enzyme selectedfor the enzyme layer can be specific to a particular analyte, or tovarious other component(s) of the in vivo environment. For example, theenzyme layer can advantageously be provided to interact with the analyteto be measured. When the analyte being measured is glucose, a layercontaining glucose oxidase can advantageously be provided. Glucoseoxidase catalyzes the conversion of oxygen and glucose to hydrogenperoxide and gluconate according to the following reaction:

Glucose+O₂→Gluconate+H₂O₂

Hydrogen peroxide produced from the catalyzed conversion of oxygen andglucose can diffuse to the surface of the reference electrode. Thehydrogen peroxide can then react with silver produced from the depletionof silver chloride in order to regenerate silver ion at the referenceelectrode surface, with the silver ion associating with in situ Cl⁻ ion.

H₂O₂+2Ag+2Cl⁻→2AgCl+H₂O+½O₂+2e ⁻

By regenerating AgCl at the reference electrode, the reference capacityof the reference electrode can be increased, thereby effectivelyincreasing the useful life of the sensor. Advantageously, the amount ofenzyme coated over a portion of the reference electrode can be carefullyselected and controlled in order to provide for a predicted/controlledreference capacity. The predicted/controlled reference capacity can beemployed to impart a longer sensor life than is observed forconventional sensors, or can be employed to impart a preselected(limited) duration of use of the sensor in vivo.

Working Electrode

In the case of certain glucose-oxidase-based glucose sensors, thespecies being measured at the working electrode is H₂O₂. Glucose oxidasecatalyzes the conversion of oxygen and glucose to hydrogen peroxide andgluconate. The change in H₂O₂ levels can be monitored to determineglucose concentration because for each glucose molecule metabolized,there is a proportional change in the product H₂O₂. Oxidation of H₂O₂ bythe working electrode is balanced by reduction of ambient oxygen, enzymegenerated H₂O₂, or other reducible species at the silver/silver chlorideelectrode. The H₂O₂ produced from the glucose oxidase reaction furtherreacts at the surface of working electrode and produces two protons(2H⁺), two electrons (24 and one oxygen molecule (O₂) (See, e.g.,Fraser, D. M. “An Introduction to In vivo Biosensing: Progress andproblems.” In “Biosensors and the Body,” D. M. Fraser, ed., 1997, pp.1-56 John Wiley and Sons, New York).

A potentiostat can be used to measure the electrochemical reaction(s) atthe electrode(s). The potentiostat applies a constant potential betweenthe working and reference electrodes to produce a current value. Thecurrent that is produced at the working electrode is proportional to thediffusional flux of H₂O₂. Accordingly, a raw signal may be produced thatis representative of the concentration of glucose in the user's body,and therefore may be utilized to estimate a meaningful glucose value,such as described elsewhere herein. As discussed elsewhere herein, insome embodiments, the working and reference electrode may be configuredto substantially prevent any hydrogen peroxide generated at thesilver/silver chloride reference electrode from migrating to the workingelectrode where it is oxidized.

Reference Electrode

In some embodiments, the reference electrode comprises asilver-containing material. The silver-containing material may includeany of a variety of materials and be in various forms, such as,silver/silver chloride polymer pastes, paints, polymer-based conductingmixture, and/or inks that are commercially available, for example. Thethird layer can be processed using a pasting/dipping/coating step, forexample, using a die-metered dip coating process. In one exemplaryembodiment, a silver/silver chloride polymer paste is applied to anelongated body by dip-coating the body (for example, using a meniscuscoating technique) and then drawing the body through a die to meter thecoating to a precise thickness. Multiple coating steps can be used tobuild up the coating to a predetermined thickness. Such a drawing methodcan be utilized for forming one or more of the electrodes in the devicedepicted in FIG. 1B.

In some embodiments, silver/silver chloride particles are mixed into apolymer, such as polyurethane, polyimide, or the like, to form thesilver-containing material for the reference electrode. In someembodiments, the material used to form the reference electrode is cured,for example, by using an oven or other curing process. A covering offluid-permeable polymer with conductive particles (such as, for example,carbon particles) therein can then be applied over the referenceelectrode.

One challenge presented by employing silver/silver chloride referenceelectrodes, such as described elsewhere herein, with glucose sensors isthat as current flows through the reference electrode, the referencecapacity decreases as the silver chloride component of the referenceelectrode is reduced to silver and chloride according to the followingreaction:

AgCl+e ⁻Ag+Cl⁻

Over time, as the silver chloride becomes completely depleted, thereference electrode potential will shift and the sensor response toglucose becomes less linear. For proper functioning of the sensor,however, the reference electrode potential should be stable throughoutthe duration of the targeted sensor life. Sensor life can therefore belimited as the silver chloride depletes and the reference potentialshifts away from the plateau potential for the analyte.

In order to extend the useful life of the sensor, a reference electrodecan be configured to have an increased or prolonged reference capacity.For example, with a silver/silver chloride reference electrode, thereference electrode can be configured such that the silver chloridewithstands depletion or otherwise becomes depleted more slowly (that is,over a greater period of time). Thus, by slowing the overall rate ofdepletion of silver chloride from the reference electrode (for example,by providing a mechanism for regenerating silver chloride), thereference capacity can remain stable (that is, linear) for a longerperiod of time. Effectively, an increase in time of the period ofstability of the reference electrode can increase the useful life of thesensor.

Advantageously, covering at least a portion of the silver/silverchloride reference electrode with an enzyme layer can increase areference capacity of a silver/silver chloride reference electrode. Forexample, in a silver/silver chloride reference electrode covered atleast in part with an enzyme that comprises glucose oxidase, after theglucose oxidase catalyzes the reaction of glucose and oxygen to generategluconate and hydrogen peroxide, the hydrogen peroxide can diffuse tothe reference electrode surface where it oxidizes silver to silver ionto regenerate silver chloride at the reference electrode surface.

As illustrated in FIGS. 17A-17D, the advantages of covering at least aportion of a reference electrode with an enzyme layer can be seen ascompared to a reference electrode without an enzyme layer. Withreference to FIGS. 17A-17D, three sets of sensors were built withsubstantially identical specifications, with the only difference beingthe surface area of the enzyme layer covering the reference electrode.The sensors were of a design corresponding to that illustrated in FIG.1C. With this design, the third layer 114 of the sensor comprised aconductive material in the form of a silver/silver chloride materialthat formed the reference electrode. As illustrated, the third layer 114was applied onto the second layer 104, which was an insulator. After thesensors were built, a membrane 108 was deposited onto the referenceelectrode 114 of two sets of sensors The membrane 108 included an enzymelayer containing glucose oxidase, which catalyzes a reaction of glucoseand oxygen to produce gluconate and hydrogen peroxide, as describedelsewhere herein. For a first set of four test sensors, no membrane (andthus no enzyme layer) was applied onto the reference electrodes. Afterall post-processing steps were completed, the sensors were placed andthen remained over time in a solution having a glucose concentration ofabout 500 mg/dL. At this concentration, the signal current continuouslygenerated by the sensor was measured to be about 20 nA. The potential ofthe reference electrodes of these four test sensors were then measuredover time. Because of a break-in time of about two hours necessary toobtain reference potential stability, measurements of referencepotential over time were compared to the reference potential at the twohour mark. As illustrated in FIG. 17A, which plots the change inreference electrode potential (compared to the potential at the two hourmark) of the four above-described test sensors as a function of time,the reference potential of these no-enzyme-covering reference electrodesdropped fairly quickly, beginning on the first day.

With reference to FIG. 17B, a second set of four test sensors were builthaving substantially identical specifications as the tests sensorscorresponding to FIG. 17A, except that the reference electrodes of thesefour test sensors were covered with an enzyme layer. The enzyme layercoverage (i.e., in terms of surface area) of the reference electrodesfor these four test sensors was about 0.00225 in². As illustrated inFIG. 17B, the reference potential of these enzyme-covering referenceelectrodes did not noticeably drop until about the eighth day after thesensor was placed in the solution.

With reference to FIG. 17C, a third set of four test sensors were builthaving substantially identical specifications as the tests sensorscorresponding to FIG. 17B, except that the reference electrodes of thesefour test sensors were covered with an enzyme layer with a surface areaof about 0.00633 in². As illustrated in FIG. 17C, the referencepotential of these enzyme-covering reference electrodes did notnoticeably drop until about the 17th day after the sensor was placed inthe solution.

With reference to FIG. 17D, the reference potentials of each of theabove-described sensor set were then averaged (i.e. arithmeticallyaveraging of four sensors in each set), and then plotted as a functionof time. As illustrated, with the reference electrodes covered at leastin part with 0.00633 in² of enzyme layer, the reference potentialremained substantially constant for a longer period of time. In fact,the reference potential of these reference electrodes did not drop untilat around 16 days. With reference electrodes covered at least in partwith 0.00225 in² of enzyme layer, the reference potential remainedsubstantially constant for a shorter period of time. The referencepotential of these reference electrodes did not drop until at around 8days. In comparison, with reference electrodes that included no enzymelayer coverage, the reference potential dropped much more quickly. Asillustrated, the reference potential of these reference electrodes beganto drop at around the first day. Accordingly, by covering at least aportion of a reference electrode with a hydrogen-peroxide generatingenzyme layer, the reference capacity of the reference electrode can begreatly increased.

In certain embodiments, the enzyme layer is configured to cover at leasta portion of a length of a reference electrode. As illustrated in FIG.17D, generally, the larger the area of the reference electrode that iscovered with an enzyme layer (that is, the longer the enzyme layercovering the reference electrode while holding all other enzyme-layerdimensions (e.g., width or depth) constant), the more the referencecapacity of the reference electrode can be increased.

In order to increase the useful life of the sensor, the length of theenzyme layer (and by extension the area of the enzyme layer) can beincreased over the length of the covering used in the standard sensorconfiguration. For example, while holding all other enzyme-layerdimensions constant, the length of the enzyme layer can be more thandoubled as compared to the length of the enzyme layer on the standardsensor, from a length of about 0.101 inches to a length of about 0.284inches, resulting in a coverage of about 60-100% of the electroactivesurface area of the reference electrode. As a result, the referencepotential remains substantially constant for a substantially longerperiod of time.

Accordingly, the percent coverage of the electroactive surface area ofthe reference electrode by the enzyme layer can be up to about 10%,about 10% or more, about 20% or more, about 30%, about 40% or more,about 50% or more, about 60% or more, about 70% or more, about 80% ormore, about 90% or more, or about 100%. Additional coverage, e.g., ofregions proximal to the reference electrode, can also be provided, andcan also generate hydrogen peroxide that can migrate to the surface ofthe reference electrode. In some embodiments, e.g., wherein a dipcoating method is employed to deposit the enzyme layer, the length ofthe enzyme layer can be about two times, about three times, about fourtimes, about five times, about six times, about seven times, about eighttimes, about nine times, about ten times, or more than about ten timesthe length of the enzyme layer employed in a working or other sensingelectrode as described herein, e.g., up to or exceeding the length ofthe in vivo portion of the analyte sensor.

The thickness of the enzyme layer, e.g., on the silver/silver chlorideof the reference electrode, can, independent of any other enzyme layer,be from about 0.01, about 0.05, about 0.6, about 0.7, or about 0.8microns to about 1, about 1.2, about 1.4, about 1.5, about 1.6, about1.8, about 2, about 2.1, about 2.2, about 2.5, about 3, about 4, about5, about 10, about 20, about 30 about 40, about 50, about 60, about 70,about 80, about 90, or about 100 microns. Preferably, the thickness ofthe enzyme layer can be from about 0.05, about 0.1, about 0.15, about0.2, about 0.25, about 0.3, about 0.35, about 0.4, about 0.45, about0.5, about 1, about 1.5, about 2, about 2.5, about 3, about 4, or about5 microns to about 6, about 7, about 8, about 9, about 10, about 11,about 12, about 13, about 14, about 15, about 16, about 17, about 18,about 19, about 19.5, about 20, about 25, or about 30 microns or more.In some embodiments, the thickness of the enzyme layer can be from about2, about 2.5, or about 3 microns to about 3.5, about 4, about 4.5, orabout 5 microns in the case of a transcutaneously implanted sensor orfrom about 6, about 7, or about 8 microns to about 9, about 10, about11, or about 12 microns in the case of a wholly implanted sensor. Whileuniform coverage may be preferred in some embodiments, in otherembodiments portions of the surface can remain uncovered, or thethickness of the enzyme layer can be irregular.

In some embodiments, the enzyme layer is configured to bring about arate of silver chloride regeneration that substantially correlates tothe rate of silver chloride depletion during a time period overdifferent levels of glucose concentration. In certain embodiments thatuse hydrogen peroxide to regenerate the reference electrode, covering atleast a portion of a reference electrode with an enzyme layer that canreact with a hydrogen peroxide-generating analyte (e.g., glucose) havinga fluctuating concentration over any period of time, can result insilver chloride being efficiently regenerated at the surface of thereference electrode at a rate that correlates with the rate of depletionof silver chloride. That is, at high analyte concentrations (e.g.,glucose concentrations), the rate of silver chloride depletion increasesas more hydrogen peroxide is produced, thereby producing more electronsas the hydrogen peroxide reacts at the electroactive surface of workingelectrode. In turn, these electrons decrease the silver chloridecomponent of the reference electrode in accordance with the reaction:

AgCl+e ⁻Ag+Cl⁻

Simultaneously, with the use of an enzyme-layer-containing membrane thatcovers the reference electrode, at these same high analyteconcentrations, the hydrogen peroxide regenerates silver chloride at anincreased rate, because of the higher concentration of hydrogenperoxide. Accordingly, a sense of balance or equilibrium is created inwhich as the rate of silver chloride depletion increases (e.g., at highanalyte concentrations) at the Ag/AgCl reference electrode, the rate ofsilver chloride regeneration simultaneously also increases. As long ashydrogen peroxide is continuously generated, silver is continuouslyregenerated to silver ion (and thus to silver chloride, the chloride ionbeing a naturally occurring constituent of the in vivo environment). Inthese embodiments, the reference electrode is continuously regenerated,with the rate of silver chloride regeneration positively correlatingwith the rate of silver chloride depletion.

As discussed elsewhere herein, the enzyme layer can be configured invivo to generate hydrogen peroxide upon exposure to a particularanalyte. For example, the enzyme layer can include glucose oxidase,which can catalyze a reaction between glucose and oxygen to generategluconate and hydrogen peroxide. Alternative or additional oxidases canbe included in the enzyme layer, including any one or more of galactoseoxidase, cholesterol oxidase, amino acid oxidase, alcohol oxidase,lactate oxidase, or uricase, for example. Moreover, alternative oradditional analytes other than glucose (e.g., urate, ascorbate, citrate,L-lactate, succinate, D-glucose, ethanol, etc.) can react with theenzyme layer to generate hydrogen peroxide.

In certain embodiments, the sensor membrane includes an enzyme layerconfigured to react with a non-measured species (i.e., one that is notbeing measured, such as, for example, uric acid) that has aconcentration that is not subject to substantial fluctuation over time,so as to ensure uninterrupted generation of a reference electroderegenerating species. For example, a species present in a host can beselected that does not substantially fluctuate in concentrationthroughout the morning, the afternoon, the evening, and/or the night, ordoes not substantially fluctuate over about a 6-hour period, over abouta 12-hour period, over about an 18-hour period, over about a 24-hourperiod, over about a 36-hour period, over about a 48-hour period, orover about a 72-hour period or more. In certain embodiments, theconcentration of the substantially constant species fluctuates less thanabout 100%, less than about 90%, less than about 80%, less than about70%, less than about 60%, less than about 50%, less than about 40%, lessthan about 30%, less than about 20%, less than about 10%, or less thanabout 5% from a maximum concentration to a minimum concentration andvice versa throughout a given period of time. Preferably, an enzymelayer is selected to react with a particular species having asubstantially constant concentration throughout a particular period oftime, for example, the analyte concentration fluctuates less than about25%, less than about 20%, less than about 15%, less than about 10%, lessthan about 5%, or less than about 1%. In some embodiments, the enzymelayer comprises at least two different enzymes, with a first enzyme(e.g., glucose oxidase) that catalyzes a reaction involving an analyte(e.g., glucose) being a reactant and hydrogen peroxide being theproduct. The second enzyme (e.g., uricase) catalyzes a reactioninvolving a non-measured species (e.g., uric acid), which is not subjectto substantial fluctuation over time, being the reactant and hydrogenperoxide being the product. In these embodiments, the rate of silverchloride regeneration may be more stable than one that entirely relieson analyte concentration. In further embodiments, the enzyme layer maycomprise a third enzyme, in which the third enzyme catalyzes a reactioninvolving another different non-measured species, which is not subjectto substantial fluctuation over time, being the reactant and hydrogenperoxide being the product. It is contemplated that such an embodimentmay further stabilize the rate of silver chloride regeneration, and makethe rate less dependent on glucose concentration. While in someembodiments, the reference electrode is substantially and continuouslyregenerated, in other embodiments, the reference electrode may beperiodically or intermittently regenerated.

Similarly, the concentration of enzyme in the enzyme layer can be anyamount suitable for generating a desired amount of hydrogen peroxide,e.g., sufficient to regenerate silver chloride, but not excess amountsthat may interfere with accurate measurements at the working electrode.Advantageously, the amount of enzyme (for example, determined by thelength and thickness of the layer and/or the concentration of enzyme inthe layer) covering the silver/silver chloride reference electrode, canbe controlled to provide for a sensor having a useful life of a knownduration. For example, the increase in sensor useful life as a functionof various amounts of enzyme layer covering on the silver/silverchloride reference electrode can be studied to determine thecorresponding relationship. Thus, when a known quantity of enzyme layeris applied to (for example, covers, coats) at least a portion of thesilver/silver reference electrode, a desired useful life of the sensor,as limited by the reference capacity of the reference electrode, can beselected. Accordingly, sensor electronics can be programmed with afailure mode that can instruct a user to change the sensor after apre-determined amount of time has passed, corresponding to the predicteduseful life of the sensor as based on the amount of enzyme applied tothe reference electrode.

In some embodiments, the material used to form the reference electrodemay be designed or selected to affect and/or control the rate ofreference electrode regeneration and/or the reference electrodecapacity. For example, the concentration of silver and silver chloridein the silver/silver chloride material (e.g., paste) can be increased incertain embodiments to increase the reference electrode capacity and/orthe rate of reference electrode regeneration. In some embodiments, thesilver/silver chloride component can form from about 10% to about 65% byweight of the total material that forms the reference electrode (i.e.,including the carrier, such as polyurethane), or from about 20% to about50%, or from about 23% to about 37%.

In addition, silver and silver chloride grains having certain particlesizes can be selectively chosen to affect and/or control the rate ofreference electrode regeneration. In certain embodiments, the silvergrain in the silver/silver chloride solution or paste has an averageparticle size corresponding to a maximum particle dimension that is lessthan about 100 microns, or less than about 50 microns, or less thanabout 30 microns, or less than about 20 microns, or less than about 10microns, or less than about 5 microns. The silver chloride grain in thesilver/silver chloride solution or paste can have an average particlesize corresponding to a maximum particle dimension that is less thanabout 100 microns, or less than about 80 microns, or less than about 60microns, or less than about 50 microns, or less than about 20 microns,or less than about 10 microns. The silver grain and the silver chloridegrain may be incorporated at a ratio of the silver chloride grain:silvergrain of from about 0.01:1 to 2:1 by weight, or from about 0.1:1 to 1:1.The silver grains and the silver chloride grains can then be mixed witha carrier (for example, a polyurethane) to form a solution or paste. Thesilver/silver chloride solution or paste can have a viscosity (underambient conditions) that can be from about 1 to about 500 centipoise, orfrom about 10 to about 300 centipoise, of from about 50 to about 150centipoise.

In some embodiments, the surface areas of the silver and silver chloridegrains in the conductive material used to form the reference electrodemay be selected to affect and/or control the rate of reference electroderegeneration and/or the reference electrode capacity. Typically, themore finely divided a reactant particle is, the faster a reactionoccurs. A finely powdered solid with numerous particles will generallyproduce a faster reaction than if the same mass is present as a singlelarge solid, all else being equal. The powdered solid has a greatersurface area than the single solid. Accordingly, as the surface area ofthe silver and silver chloride grains in the silver/silver chlorideconductive material is increased, the rate of silver chlorideregeneration in the reference electrode may also be increased, all elsebeing equal. In some embodiments, the average surface area per weight(or unit volume) of the silver grains in the reference electrode priorto use is from about 0.5 m²/gm to about 5 m²/gm, from about 0.7 m²/gm toabout 3.5 m²/gm, from 0.75 m²/gm to about 2.5 m²/gm, or from about 1m²/gm to about 1.5 m²/gm. In certain embodiments, the average surfacearea per unit volume of the silver chloride grains in the referenceelectrode prior to use is from about 0.1 m²/gm to about 0.9 m²/gm, fromabout 0.2 m²/gm to about 0.7 m²/gm, from about 0.3 m²/gm to about 0.6m²/gm, or from about 0.4 m²/gm to about 0.6 m²/gm.

Exemplary Analyte Sensors, Electrode Configurations, and Methods ofManufacturing Same

FIG. 2C is an expanded view of an exemplary embodiment of a continuousanalyte sensor, also referred to as an analyte sensor, illustrating thesensing mechanism. In some embodiments, the sensing mechanism is adaptedfor insertion under the host's skin, and the remaining body of thesensor (including, for example, electronics) can reside ex vivo. In theillustrated embodiment, the analyte sensor includes two electrodes, suchas, for example, a working electrode 112 and at least one additionalelectrode 114. The additional electrode may be a silver/silver chloridereference electrode; however it is contemplated that other types ofreference electrodes can be employed.

It is contemplated that the electrode(s) can be formed to have any of avariety of cross-sectional shapes. For example, in some embodiments, theelectrode may be formed to have a circular or substantially circularcross-sectional shape, for example, as illustrated in FIGS. 2A-2C.Alternatively, the electrode may be formed to have a cross-sectionalshape that resembles an ellipse, a polygon (such as, for example,triangle, square, rectangle, parallelogram, trapezoid, pentagon,hexagon, or octagon), or the like. The cross-sectional shape of theelectrode can be symmetrical. Alternatively, the cross-sectional shapecan be asymmetrical. In some embodiments, each electrode can be formedfrom a fine wire with a diameter of from about 0.001 inches or less toabout 0.05 inches or more. Each electrode can be formed from, forexample, a plated insulator, a plated wire, or bulk electricallyconductive material. In some embodiments, the wire used to form aworking electrode may be about 0.002 inches, about 0.003 inches, about0.004 inches, about 0.005 inches, about 0.006 inches, about 0.007inches, about 0.008 inches, about 0.009 inches, about 0.01 inches, about0.015 inches, about 0.02 inches, about 0.025 inches, about 0.03 inches,about 0.035 inches, about 0.04 inches, or about 0.045 inches indiameter. In some embodiments, the working electrode can comprise a wireformed from a conductive material, such as platinum, platinum-black,platinum-iridium, palladium, graphite, gold, carbon, conductive polymer,alloys, or the like. Any alternate sensor configurations can be employedwith the analyte sensor system as described herein.

The working electrode 112 can be configured to measure the concentrationof an analyte, such as, but not limited, to glucose, uric acid,cholesterol, lactate, and the like. In an enzymatic electrochemicalsensor for detecting glucose, for example, the working electrode canmeasure the hydrogen peroxide produced by an enzyme-catalyzed reactionof the analyte being detected, which can create a measurable electriccurrent. For example, in the detection of glucose, glucose oxidase (GOx)produces, inter alia, hydrogen peroxide (H₂O₂) as a byproduct. The H₂O₂can react with the surface of the working electrode to produce twoprotons (2H⁺), two electrons (2e⁻) and one molecule of oxygen (O₂),which produces the electric current being detected.

Moreover, an insulator can be provided to electrically insulate theworking and reference electrodes. For example, the working electrode 112can be covered with an insulating material, such as a non-conductivepolymer. Dip-coating, spray-coating, vapor-deposition, or other coatingor deposition techniques can be used to deposit the insulating materialon the working electrode. In some embodiments, the insulating materialcomprises parylene. Parylene can be an advantageous polymer coatingbecause of its strength, lubricity, and electrical insulationproperties. Generally, parylene is produced by vapor deposition andpolymerization of para-xylylene (or its substituted derivatives). Anysuitable insulating material can be used, including, for example,fluorinated polymers, polyethyleneterephthalate, polyurethane,polyimide, other nonconducting polymers, or the like. Glass or ceramicmaterials can also be employed. Other materials suitable for use includesurface energy modified coating systems such as those marketed under thetrade names AMC18, AMC148, AMC141, and AMC321 by Advanced MaterialsComponents Express of Bellafonte, Pa. In some embodiments, the workingelectrode may not require a coating of insulator.

In some embodiments, the additional electrode 114 is configured tofunction as a reference electrode alone. When employed as a referenceelectrode, the additional electrode 114 can be a silver/silver chlorideelectrode. In some embodiments, the electrodes are juxtapositioned ortwisted with or around each other. Other configurations can also beused, as described elsewhere herein. In some embodiments, the referenceelectrode 114 is helically wound around the working electrode 112. Theassembly of wires can then be optionally coated together with aninsulating material, similar to that described above, in order toprovide an insulating attachment (for example, securing together of theworking and reference electrodes).

In some embodiments, a radial window is formed through the insulatingmaterial to expose a circumferential electroactive surface of theworking electrode 112. Additionally, sections of electroactive surfaceof the reference electrode can be exposed. For example, the sections ofelectroactive surface can be masked during deposition of an outerinsulating layer or etched after deposition of an outer insulatinglayer. In some applications, cellular attack or migration of cells tothe sensor can cause reduced sensitivity or function of the device,particularly after the first day of implantation. When the exposedelectroactive surface is distributed circumferentially about the sensor(for example, as in a radial window), the available surface area forreaction can be sufficiently distributed so as to minimize the effect oflocal cellular invasion of the sensor on the sensor signal.Alternatively, a tangential exposed electroactive window can be formed,for example, by stripping only one side of the coated assemblystructure. In other alternative embodiments, the window can be providedat the tip of the coated assembly structure such that the electroactivesurfaces are exposed at the tip of the sensor. Other methods andconfigurations for exposing electroactive surfaces can also be employed.

In some embodiments, wherein the sensor comprises two workingelectrodes, the two working electrodes can be juxtapositioned, aroundwhich the reference electrode can be disposed (for example, helicallywound). In some embodiments with two or more working electrodes, theworking electrodes can be formed in a double-, triple-, quad-, etc.helix configuration along the length of the sensor (for example,surrounding a reference electrode, insulated rod, or other supportstructure). The resulting electrode system can be configured with anappropriate membrane system, wherein the first working electrode can beconfigured to measure a first signal comprising glucose and baselinesignals, and the additional working electrode can be configured tomeasure a baseline signal consisting of the baseline signal only. Inthese embodiments, the second working electrode can be configured to besubstantially similar to the first working electrode, but without anenzyme disposed thereon. In this way, the baseline signal can bedetermined and subtracted from the first signal to generate a differencesignal, that is, a glucose-only signal that is substantially not subjectto fluctuations in the baseline or interfering species on the signal,such as described in U.S. Pat. No. 7,715,893; U.S. Pat. No. 7,460,898;U.S. Pat. No. 7,761,130; and U.S. Patent Publication No.2008-0083617-A1, each of which are incorporated herein by reference intheir entirety.

In some embodiments, the sensor is configured and arranged forimplantation in a host and for generating in vivo a signal associatedwith an analyte in a sample of the host during a sensor session. Forexample, the sensor can be configured to generate in vivo a signalassociated with a glucose concentration of the host during a sensorsession. The length of time of the sensor session can be from less thanabout 10 minutes, to about 10 minutes or more, about 20 minutes or more,about 30 minutes or more, about 40 minutes or more, or about 50 minutesor more, about 1 hour or more, about 2 hours or more, about 3 hours ormore, about 4 hours or more, or about 5 hours or more. In someembodiments, the time length of the sensor session can be from about 1hour or more to about 2 hours or more, about 3 hours or more, about 4hours or more, about 5 hours or more, about 6 hours or more, about 7hours or more, about 8 hours or more, about 9 hours or more, about 10hours or more, about 11 hours or more, about 12 hours or more, about 13hours or more, about 14 hours or more, about 15 hours or more, about 16hours or more, about 17 hours or more, about 18 hours or more, about 19hours or more, about 20 hours or more, about 21 hours or more, about 22hours or more, about 23 hours or more, or about 24 hours or more. Insome embodiments, the time length of the sensor session can be from lessthan about 0.25 days to about 0.25 days or more, about 0.5 days or moreabout 0.75 days or more, about 1 day or more, about 2 days or more,about 3 days or more, about 4 days or more, about 5 days or more, about6 days or more, about 7 days or more, about 8 days or more, about 9 daysor more, about 10 days or more, about 20 days or more, about 30 days ormore, about 40 days or more, or about 50 days or more.

The analyte sensor can be configured for any type of implantation, suchas transcutaneous implantation, subcutaneous implantation, orimplantation into the host's circulatory system (for example, into avessel, such as a vein or artery). In addition, the sensor may beconfigured to be wholly implantable or extracorporeally implantable (forexample, into an extracorporeal blood circulatory device, such as aheart-bypass machine or a blood dialysis machine). U.S. Pat. No.7,497,827 describes an exemplary continuous analyte sensor that can beused for transcutaneous implantation by insertion into the abdominaltissue of a host. U.S. Patent Publication No. 2008-0119703-A1 describesan exemplary embodiment of a continuous analyte sensor that can be usedfor insertion into a host's vein (for example, via a catheter). In someembodiments, the sensor can be configured and arranged for in vitro use.

FIG. 6A is a cross-sectional view through the sensor of FIG. 2C on line6-6, illustrating one embodiment of the membrane system 612. Asillustrated, the membrane system can include an enzyme domain 602, adiffusion resistance domain 604, and a bioprotective domain 606 locatedaround the working electrode 602. In some embodiments, a unitarydiffusion resistance domain and bioprotective domain can be included inthe membrane system (for example, wherein the functionality of bothdomains is incorporated into one domain, that is, the bioprotectivedomain). In some embodiments, the sensor is configured for short-termimplantation (for example, from about 1 to about 30 days). However, itis understood that the membrane system 612 can be modified for use inother devices, for example, by including only one or more of thedomains, or additional domains.

In some embodiments, the membrane system can include a bioprotectivedomain, also referred to as a cell-impermeable domain or biointerfacedomain, comprising a surface-modified base polymer as described in moredetail elsewhere herein. The membrane systems 612 of some embodimentscan also include a plurality of domains or layers including, forexample, an electrode domain 610 (for example, as illustrated in FIG.6C), an interference domain 608 (for example, as illustrated in FIG.6B), or a cell disruptive domain (not shown), such as described in moredetail elsewhere herein and in U.S. Pat. No. 7,494,465, which isincorporated herein by reference in its entirety.

Membranes modified for other sensors or electrodes, for example, mayinclude fewer or additional layers. For example, the membrane system cancomprise one electrode layer, one enzyme layer, and two bioprotectivelayers. Alternatively, the membrane system can comprise one electrodelayer, two enzyme layers, and one bioprotective layer. Furthermore, thebioprotective layer can be configured to function as the diffusionresistance domain and control the flux of the analyte (such as, forexample, glucose) to the underlying membrane layers.

In the case of a silver/silver chloride reference electrode, one or moreenzyme layers can be employed. It is generally preferred to avoidadditional layers that would block reactants that generate hydrogenperoxide upon contact with the enzyme layer; however, such layers may bepresent, e.g., for ease of fabrication, provided that sufficientreactant reaches the enzyme layer so as to generate hydrogen peroxide toregenerate silver chloride.

In some embodiments, one or more domains of the sensing membranes can beformed from materials such as silicone, polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutyleneterephthalate (PBT), polymethylmethacrylate (PMMA), polyether etherketone (PEEK), polyurethanes, cellulosic polymers, poly(ethylene oxide),poly(propylene oxide) and copolymers and blends thereof, polysulfonesand block copolymers thereof including, for example, di-block,tri-block, alternating, random and graft copolymers.

In some embodiments, various domains or layers, including the enzymelayer, can be deposited on the electroactive surfaces of the electrodematerial using known thin or thick film techniques (for example,spraying, electro-depositing, dipping, or the like). It should beappreciated that the enzyme layer located over the working electrodedoes not have to have the same structure or composition as the enzymelayer located over, e.g., a silver/silver chloride reference electrode.As described in greater detail elsewhere herein, however, a membrane orother layer including an enzyme domain deposited over the workingelectrode can advantageously be deposited over the silver/silverchloride reference electrode as well, to increase or control referencecapacity via action of the enzyme therein as described above.

Although the exemplary embodiments illustrated in FIGS. 6A-6C involvecircumferentially extending membrane systems, the membranes describedherein may be applied to any planar or non-planar surface, for example,the substrate-based sensor structure of U.S. Pat. No. 6,565,509 to Sayet al.

In some embodiments, an enzyme domain, also referred to as the enzymelayer, can be used and can be situated less distal from theelectrochemically reactive surfaces than the diffusion resistancedomain. The enzyme domain can comprise a catalyst or enzyme configuredto react with an analyte. For example, the enzyme domain can be animmobilized enzyme domain including glucose oxidase. In otherembodiments, the enzyme domain can be impregnated with other oxidases,including, for example, galactose oxidase, cholesterol oxidase, aminoacid oxidase, alcohol oxidase, lactate oxidase, or uricase. For anenzyme-based electrochemical glucose sensor to perform well, thesensor's response should not be limited by either enzyme activity orcofactor concentration.

In some embodiments, the catalyst or enzyme can be impregnated orotherwise immobilized into the bioprotective or diffusion resistancedomain such that a separate enzyme domain is not required (for example,wherein a unitary domain is provided including the functionality of thebioprotective domain, diffusion resistance domain, and enzyme domain).In some embodiments, the enzyme domain is formed from a polyurethane,for example, aqueous dispersions of colloidal polyurethane polymersincluding the enzyme.

FIGS. 1A-1C illustrate alternative embodiments of the in vivo portion ofa continuous analyte sensor 100, which includes an elongated conductivebody 102. The elongated conductive body 102 includes a core 110 (seeFIG. 1B) and a first layer 112 at least partially surrounding the core.The first layer includes a working electrode (for example, located inwindow 106) and a membrane 108 located over the working electrode. Insome embodiments, the core and first layer can be of a single material(such as, for example, platinum). In some embodiments, the elongatedconductive body is a composite of at least two materials, such as acomposite of two conductive materials, or a composite of at least oneconductive material and at least one non-conductive material. In someembodiments, the elongated conductive body comprises a plurality oflayers. In certain embodiments, there are at least two concentric orannular layers, such as a core formed of a first material and a firstlayer formed of a second material. However, additional layers can beincluded in some embodiments. In some embodiments, the layers arecoaxial.

The elongated conductive body may be long and thin, yet flexible andstrong. For example, in some embodiments, the smallest dimension of theelongated conductive body is less than about 0.1 inches, 0.075 inches,0.05 inches, 0.025 inches, 0.01 inches, 0.004 inches, or 0.002 inches.While the elongated conductive body is illustrated in FIGS. 1A through1C as having a circular cross-section, in other embodiments thecross-section of the elongated conductive body can be ovoid,rectangular, triangular, polyhedral, star-shaped, C-shaped, T-shaped,X-shaped, Y-Shaped, irregular, or the like. In one embodiment, aconductive wire electrode is employed as a core. To such a cladelectrode, two additional conducting layers may be added (e.g., withintervening insulating layers provided for electrical isolation). Theconductive layers can be comprised of any suitable material. In certainembodiments, it can be desirable to employ a conductive layer comprisingconductive particles (i.e., particles of a conductive material) in apolymer or other binder.

The materials used to form the elongated conductive body (such as, forexample, stainless steel, titanium, tantalum, platinum,platinum-iridium, iridium, certain polymers, and/or the like) can bestrong and hard, and therefore are resistant to breakage. In someembodiments, the sensor's small diameter provides flexibility to thesematerials, and therefore to the sensor as a whole. Thus, the sensor canwithstand repeated forces applied to it by surrounding tissue.

In addition to providing structural support, resiliency and flexibility,in some embodiments, the core 110, or a component thereof, provideselectrical conduction for an electrical signal from the workingelectrode to sensor electronics (not shown). In some embodiments, thecore 110 comprises a conductive material, such as stainless steel,titanium, tantalum, a conductive polymer, and/or the like. However, inother embodiments, the core is formed from a non-conductive material,such as a non-conductive polymer. In yet other embodiments, the corecomprises a plurality of layers of materials. For example, in oneembodiment the core includes an inner core and an outer core. In afurther embodiment, the inner core is formed of a first conductivematerial and the outer core is formed of a second conductive material.For example, in some embodiments, the first conductive material isstainless steel, titanium, tantalum, a conductive polymer, an alloy,and/or the like, and the second conductive material is a conductivematerial selected to provide electrical conduction between the core andthe first layer, and/or to attach the first layer to the core (that is,if the first layer is formed of a material that does not attach well tothe core material). In another embodiment, the core is formed of anon-conductive material (such as, for example, a non-conductive metaland/or a non-conductive polymer) and the first layer is formed of aconductive material, such as stainless steel, titanium, tantalum, aconductive polymer, and/or the like. The core and the first layer can beof a single (or same) material, such as platinum. One skilled in the artappreciates that additional configurations are possible.

Referring again to FIGS. 1A-1C, the first layer 112 can be formed of aconductive material and the working electrode can be an exposed portionof the surface of the first layer 112. Accordingly, the first layer 112can be formed of a material configured to provide a suitableelectroactive surface for the working electrode, a material such as, butnot limited to, platinum, platinum-iridium, gold, palladium, iridium,graphite, carbon, a conductive polymer, an alloy and/or the like.

As illustrated in FIGS. 1B-1C, a second layer 104 surrounds at least aportion of the first layer 112, thereby defining the boundaries of theworking electrode. In some embodiments, the second layer 104 serves asan insulator and is formed of an insulating material, such as polyimide,polyurethane, parylene, or any other known insulating materials. Forexample, in one embodiment the second layer is disposed on the firstlayer and configured such that the working electrode is exposed viawindow 106. In some embodiments, an elongated conductive body, includingthe core, the first layer and the second layer, is provided. A portionof the second layer can be removed to form a window 106, through whichthe electroactive surface of the working electrode (that is, the exposedsurface of the first layer 112) is exposed. In some embodiments, aportion of the second and (optionally) third layers can be removed toform the window 106, thus exposing the working electrode. Removal ofcoating materials from one or more layers of the elongated conductivebody (for example, to expose the electroactive surface of the workingelectrode) can be performed by hand, excimer lasing, chemical etching,laser ablation, grit-blasting, or the like.

The sensor can further comprise a third layer 114 comprising aconductive material. For example, the third layer 114 may comprise areference electrode, which may be formed of a silver-containing materialthat is applied onto the second layer 104 (that is, the insulator). Amore detailed description of the various embodiments of the referenceelectrode is described elsewhere herein.

The elongated conductive body 102 can further comprise one or moreintermediate layers (not shown) located between the core 110 and thefirst layer 112. For example, the intermediate layer can be one or moreof an insulator, a conductor, a polymer, and/or an adhesive.

It is contemplated that the ratio between the thickness of thesilver/silver chloride layer and the thickness of an insulator (such as,for example, polyurethane or polyimide) layer can be controlled, so asto allow for a certain error margin (that is, an error margin associatedwith the etching process) that would not result in a defective sensor(for example, due to a defect resulting from an etching process thatcuts into a depth more than intended, thereby unintentionally exposingan electroactive surface). This ratio may be different depending on thetype of etching process used, whether it is laser ablation, gritblasting, chemical etching, or some other etching method. In oneembodiment in which laser ablation is performed to remove asilver/silver chloride layer and a polyurethane layer, the ratio of thethickness of the silver/silver chloride layer and the thickness of thepolyurethane layer can be from about 1:5 to about 1:1, or from about 1:3to about 1:2.

In some embodiments, the core 110 comprises a non-conductive polymer andthe first layer 112 comprises a conductive material. Such a sensorconfiguration can advantageously provide reduced material costs, in thatit replaces a typically expensive material with an inexpensive material.For example, the core 110 can be formed of a non-conductive polymer,such as, a nylon or polyester filament, string or cord, which can becoated and/or plated with a conductive material, such as platinum,platinum-iridium, gold, palladium, iridium, graphite, carbon, aconductive polymer, and allows or combinations thereof.

As illustrated in FIGS. 1C-1D, the sensor can also include a membrane108, such as those discussed elsewhere herein, for example, withreference to FIGS. 6A-6C. The membrane 108 can include an enzyme layer(not shown), as described elsewhere herein. For example, the enzymelayer can include a catalyst or enzyme configured to react with ananalyte. For example, the enzyme layer can be an immobilized enzymelayer including glucose oxidase. In other embodiments, the enzyme layercan be impregnated with other oxidases, including, for example,galactose oxidase, cholesterol oxidase, amino acid oxidase, alcoholoxidase, lactate oxidase, or uricase.

The enzyme reacts with the analyte and/or another species to generatehydrogen peroxide at the reference electrode surface. For example, anenzyme layer including glucose oxidase can catalyze the reaction ofglucose and oxygen to generate gluconate and hydrogen peroxide asdescribed elsewhere herein, or a uricase oxidase can be employed togenerate hydrogen peroxide by reaction with urea. The hydrogen peroxidethus generated can diffuse to the surface of the reference electrode toregenerate silver chloride, thereby increasing the reference capacity ofthe reference electrode.

The membrane 108, including the enzyme layer, can cover at least aportion of the working and reference electrodes. By covering at least aportion of the working electrode with the membrane 108, for example, theenzyme layer can react with the analyte to be measured. Advantageously,the enzyme can be very specific to a particular analyte. Additionally,when the analyte itself is not sufficiently electro-active, the enzymecan be used to interact with the analyte to generate another specieswhich is electro-active and to which the sensor can produce a desiredoutput. For example, when the analyte being measured is glucose, aglucose oxidase enzyme layer can be provided in the membrane 108 tocatalyze the conversion of glucose and oxygen into gluconate andhydrogen peroxide. Hydrogen peroxide can then be qualified or quantifiedat the working electrode to determine a concentration of glucose at theworking electrode.

Moreover, by covering at least a portion of the reference electrode withthe membrane 108, including the enzyme layer, silver chloride at thesurface of the reference electrode can be regenerated to advantageouslyincrease reference capacity, and thereby increase the useful life of thesensor. For example, an enzyme can be included in the enzyme layer thatwill catalyze the conversion of oxygen and an analyte to generate atleast hydrogen peroxide. In some embodiments, glucose oxidase can beincluded in the enzyme layer to convert glucose and oxygen into hydrogenperoxide and gluconate. Hydrogen peroxide can then reduce silver ion tosilver metal, which can react with chloride ions to regenerate silverchloride at the reference electrode surface. Advantageously, because theenzyme layer covers at least a portion of the reference electrode, bymeans of the membrane 108, hydrogen peroxide can more efficientlyregenerate silver chloride at the surface of the reference electrode.For example, by covering at least a portion of the reference electrodewith the membrane 108, and thus the enzyme layer, hydrogen peroxide willbe generated in the vicinity of the reference electrode, thus reducingthe risk that the hydrogen peroxide will either not diffuse towards thereference electrode, or will diffuse away from the reference electrode.That is, generating hydrogen peroxide in the vicinity of the referenceelectrode can ensure that at least some hydrogen peroxide will beavailable to reduce silver ion to silver metal in order to regeneratesilver chloride at the reference electrode surface.

FIG. 1B is a schematic illustrating an embodiment of an elongatedconductive body 102, or elongated body, wherein the elongated conductivebody is formed from at least two materials and/or layers of conductivematerial, as described in greater detail elsewhere herein. The term“electrode” can be used herein to refer to the elongated conductivebody, which includes the electroactive surface that detects the analyte.In some embodiments, the elongated conductive body provides anelectrical connection between the electroactive surface (that is, theworking electrode) and the sensor electronics (not shown). In certainembodiments, each electrode (that is, the elongated conductive body onwhich the electroactive surface is located) is formed from a fine wirewith a diameter of from about 0.001 inches or less to about 0.01 inchesor more. Each electrode can be formed from, for example, a platedinsulator, a plated wire, or bulk electrically conductive material. Forexample, in some embodiments, the wire and/or elongated conductive bodyused to form a working electrode is about 0.002, 0.003, 0.004, 0.005,0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04or 0.045 inches in diameter.

Furthermore, the first layer can comprise an electroactive surface (thatis, the portion exposed through the window 106). The exposedelectroactive surface can be the working electrode. For example, if thesensor is an enzymatic electrochemical analyte sensor, the analyteenzymatically reacts with an enzyme in the membrane covering at least aportion of the electroactive surface. The reaction can generateelectrons (e) that are detected at the electroactive surface as ameasurable electronic current. For example, in the detection of glucosewherein glucose oxidase produces hydrogen peroxide as a byproduct,hydrogen peroxide reacts with the surface of the working electrodeproducing two protons (2H⁺), two electrons (2e⁻) and one molecule ofoxygen (O₂), which produces the electronic current being detected.

As previously described with reference to FIG. 1A and as illustrated inFIG. 1C, an insulator 104 is disposed on at least a portion of theelongated conductive body 102. In some embodiments, the sensor isconfigured and arranged such that the elongated body includes a core 110and a first layer 112, and a portion of the first layer 112 is exposedvia window 106 in the insulator 104. In other embodiments, the sensor isconfigured and arranged such that the elongated body 102 includes a core110 embedded in an insulator 104, and a portion of the core 110 isexposed via the window 106 in the insulator 104. For example, theinsulating material can be applied to the elongated body 102 (by, forexample, screen-, ink-jet and/or block-print) in a configurationdesigned to leave at least a portion of the first layer's 112 surface(or the core's 110 surface) exposed. For example, the insulatingmaterial can be printed in a pattern that does not cover a portion ofthe elongated body 102. Alternatively, a portion of the elongated body102 can be masked prior to application of the insulating material.Removal of the mask, after insulating material application, can exposethe portion of the elongated body 102.

In some embodiments, the insulating material 104 comprises a polymer,for example, a non-conductive (that is, dielectric) polymer.Dip-coating, spray-coating, vapor-deposition, printing and/or other thinfilm and/or thick film coating or deposition techniques can be used todeposit the insulating material on the elongated body 102 and/or core110. For example, in some embodiments, the insulating material isapplied as a layer of from about less than 5 microns, or from 5, 10 or15-microns to about 20, 25, 30 or 35-microns or more in thickness. Theinsulator can be applied as a single layer of material, or as two ormore layers, which are comprised of either the same or differentmaterials, as described elsewhere herein. Alternatively, the conductivecore may not require a coating of insulator. In some embodiments, theinsulating material defines an electroactive surface of the analytesensor (that is, the working electrode). For example, a surface of theconductive core (such as, for example, a portion of the first layer 112)can either remain exposed during the insulator application, or a portionof applied insulator can be removed to expose a portion of theconductive core's surface, as described above.

In some embodiments, in which the sensor has an insulated elongated bodyor an insulator disposed upon a conductive structure, a portion of theinsulating material can be stripped or otherwise removed, for example,by hand, excimer lasing, chemical etching, laser ablation, grit-blasting(such as, for example, with sodium bicarbonate or other suitable grit),or the like, to expose the electroactive surfaces. In one exemplaryembodiment, grit blasting is implemented to expose the electroactivesurface(s), for example, by utilizing a grit material that issufficiently hard to ablate the polymer material yet also sufficientlysoft so as to minimize or avoid damage to the underlying metal electrode(for example, a platinum electrode). Although a variety of “grit”materials can be used (such as, for example, sand, talc, walnut shell,ground plastic, sea salt, and the like), in some embodiments, sodiumbicarbonate is an advantageous grit-material because it is sufficientlyhard to ablate, e.g., a parylene coating without damaging, e.g., anunderlying platinum conductor. An additional advantage of sodiumbicarbonate blasting includes its polishing action on the metal as itstrips the polymer layer, thereby eliminating a cleaning step that mightotherwise be necessary. Alternatively, a portion of an electrode orother conductive body can be masked prior to depositing the insulator inorder to maintain an exposed electroactive surface area.

The electroactive surface of the working electrode can be exposed byformation of a window 106 in the insulator 104. The electroactive window106 of the working electrode can be configured to measure theconcentration of an analyte.

The reference electrode can be juxtapositioned and/or twisted with oraround at least a portion of the sensor, and can then optionally becoated or adhered together with an insulating material, similar to thatdescribed above, so as to provide an insulating attachment.

In some embodiments, a silver wire is formed onto and/or fabricated intothe sensor and subsequently chloridized to form a silver/silver chloridereference electrode. Advantageously, chloridizing the silver wire asdescribed herein enables the manufacture of a reference electrode withgood in vivo performance. By controlling the quantity and amount ofchloridization of the silver to form silver/silver chloride, improvedbreak-in time, stability of the reference electrode and extended lifecan be obtained in some embodiments. Additionally, use of silverchloride as described above allows for relatively inexpensive and simplemanufacture of the reference electrode.

Referring to FIGS. 1B-1C, the reference electrode 114 can comprise asilver-containing material (e.g., silver/silver chloride) applied overat least a portion of the insulating material 104, as discussed ingreater detail elsewhere herein. For example, the silver-containingmaterial can be applied using thin film and/or thick film techniques,such as but not limited to dipping, spraying, printing,electro-depositing, vapor deposition, spin coating, and sputterdeposition, as described elsewhere herein. For example, a silver orsilver chloride-containing paint (or similar formulation) can be appliedto a reel of the insulated conductive core. Alternatively, the reel ofinsulated elongated body (or core) is cut into single unit pieces (thatis, “singularized”), and silver-containing ink is pad printed thereon.In still other embodiments, the silver-containing material is applied asa silver foil. For example, an adhesive can be applied to an insulatedelongated body, around which the silver foil can then be wrapped in.Alternatively, the sensor can be rolled in Ag/AgCl particles, such thata sufficient amount of silver sticks to and/or embeds into and/orotherwise adheres to the adhesive for the particles to function as thereference electrode. In some embodiments, the sensor's referenceelectrode includes a sufficient amount of chloridized silver that thesensor measures and/or detects the analyte for at least three days.

FIG. 5A is a perspective view of the in vivo portion a dual-electrodeanalyte sensor, in another embodiment. As illustrated, the sensor cabcomprises first and second bundled elongated bodies (that is, conductivecores) E1, E2, wherein a working electrode comprises an exposedelectroactive surface of the elongated body, and a reference electrode114, wherein each working electrode comprises a conductive core. Forexample, the first working electrode comprises an exposed portion of thesurface of a first elongated body 102A having an insulating material104A disposed thereon, such that the portion of the surface of theelongated body (that is, the working electrode) is exposed via a radialwindow 106A in the insulator. In some embodiments, the elongated bodycomprises a core and a first layer, wherein an exposed surface (e.g.,electroactive) of the first layer is the first working electrode. Thesecond working electrode comprises an exposed surface of a second core102B having an insulator 104B disposed thereon, such that a portion ofthe surface of the core is exposed via a radial window 106B in theinsulator. A first layer (not shown) can be applied to the exposedsurface of the second core to form the second working electrode. In thisembodiment, the radial windows are spaced such that the workingelectrodes (that is, electroactive surfaces) are substantiallyoverlapping along the length of the sensor. Alternatively, the workingelectrodes can be spaced such that they are not substantiallyoverlapping along the length of the sensor. In this embodiment, thereference electrode comprises a wire (such as, for example, Ag/AgClwire) wrapped around the bundled conductive cores. However, in someembodiments, the reference electrode comprises a layer ofsilver-containing material applied to at least one of the insulatingmaterials 104A, 104B.

FIG. 5B is a perspective view of the in vivo portion of a dual-electrodeanalyte sensor, in another embodiment. As illustrated, the first andsecond elongated bodies E1, E2 can be twisted into a twisted pair, suchas a helix. In the embodiment shown in FIG. 5B, the reference electrode114 can then be wrapped around the twisted pair. However, in someembodiments, the reference electrode comprises a layer ofsilver-containing material applied to at least one of the insulatingmaterials 104A, 104B.

FIGS. 5C and 5D include views of the in vivo portion of a dual-electrodeanalyte sensor, in additional embodiments. As illustrated, the first andsecond elongated bodies E1, E2 can be bundled together with thereference electrode 114. Connectors 502 can be configured and arrangedto hold the conductive cores and reference electrode together.Alternatively, instead of connectors 502, a tube 530 or heat shrinkmaterial can be employed as a connector and/or supporting member. Thetubing or heat shrink material may include an adhesive inside the tubeso as to provide enhanced adhesion to the components secured within(that is, wire(s), core, layer materials, etc.). In such aconfiguration, the heat-shrink material functions not only as aninsulator, but also to hold the proximal ends of the sensor together soas to prevent or reduce fatigue and/or to maintain the electrodestogether in the event of a fatigue failure. In the embodiment depictedin FIG. 5C, the wires need not be a core and a layer, but can insteadcomprise bulk materials. The distal ends of the sensor can be loose andfinger-like, as depicted in FIG. 5C, or can be held together with an endcap. A reference electrode can be placed on one or more of the first andsecond elongated bodies instead of being provided as a separateelectrode, and the first and second elongated bodies including at leastone reference electrode thereof can be bundled together. Heat shrinktubing, crimp wrapping, dipping, or the like can be employed to bundleone or more elongated bodies together. In some embodiments, thereference electrode is a wire, such as described elsewhere herein. Inother embodiments, the reference electrode comprises a foil. In anembodiment of a dual-electrode analyte sensor, the first and secondelongated bodies can be present as or formed into a twisted pair, whichis subsequently bundled with a wire or foil reference electrode.Connectors, which can also function as supporting members, can beconfigured and arranged to hold the conductive cores and referenceelectrode together. Although in the embodiment shown in FIG. 5C, thereference electrode 114 comprise an elongated body that is bundledtogether with first and second elongated bodies E1, E2, in otherembodiments, the reference electrode may be formed of a layer ofsilver-containing material applied to at least one of the insulatingmaterials 504A, 504B.

A membrane (not shown), as described more fully elsewhere herein, canalso be included with the sensors shown in FIGS. 5A-5D. The membrane caninclude an enzyme layer as described elsewhere herein. For example, theenzyme layer can include a catalyst or enzyme configured to react withan analyte. For example, the enzyme layer can be an immobilized enzymelayer including glucose oxidase. In other embodiments, the enzyme layercan be impregnated with other oxidases, including, for example,galactose oxidase, cholesterol oxidase, amino acid oxidase, alcoholoxidase, lactate oxidase, or uricase.

FIGS. 8A-8C provide views of the in vivo portion of another embodimentof a multi-electrode sensor system 800 comprising two working electrodesand at least one reference electrode. The sensor system 800 can comprisefirst and second elongated bodies E1, E2, each formed of a conductivecore or of a core with a conductive layer deposited thereon. Asillustrated, an insulating layer 810, a conductive layer 820, and amembrane layer (not shown) can be deposited on top of the elongatedbodies E1, E2. The insulating layer 810 can separate the conductivelayer 820 from the elongated body. The materials selected to form theinsulating layer 810 can include any of the insulating materialsdescribed elsewhere herein, including, for example, polyurethane andpolyimide. The materials selected to form the conductive layer 820 caninclude any of the conductive materials described elsewhere herein,including, for example, silver/silver chloride, platinum, gold, can thelike. Working electrodes 802′, 802″ can be formed by removing portionsof the conductive layer 820 and the insulating layer 810, therebyexposing an electroactive surface of the elongated bodies E1, E2,respectively. By depositing a membrane layer comprising an enzyme on topof the elongated bodies E1, E2, the enzyme can cover at least a portionof the working and reference electrodes. Although not shown in FIGS.6A-6C, the distal ends 830′, 830″ of the core portions of the elongatedbodies E1, E2, respectively, can be covered with an insulating material(such as, for example, polyurethane or polyimide). Alternatively, theexposed core portions 830′, 830″ can be covered with a membrane systemand serve as additional working electrode surface area. Contacts 804′,804″ are used to provide electrical connection between the workingelectrodes and other components of the sensor system may be formed in asimilar manner. As shown, contacts 804′ and 804″ are separated from eachother to prevent an electrical connection therebetween. Because thelayer removal process is performed on each individual elongated body E1,E2, instead of a single geometrically complicated elongated body, thisparticular sensor design (i.e., two elongated bodies placed side byside) may provide ease of manufacturing, as compared to themanufacturing processes involved with other multi-electrode systemshaving other geometries.

The two elongated bodies illustrated in FIG. 8A can be fabricated tohave substantially the same shape and dimensions. For example, theworking electrodes can be fabricated to have the same properties,thereby providing a sensor system capable of providing redundancy ofsignal measurements. In other embodiments, the working electrodes,associated with the elongated bodies E1, E2, can each have one or morecharacteristics that distinguish each working electrode from the other.For example, in one embodiment, each of the elongated bodies E1, E2 canbe covered with a different membrane so that each working electrode canhave a different membrane property than the other working electrode. Forexample, one of the working electrodes can have a membrane comprising anenzyme layer and the other working electrode can have a membranecomprising a layer having either an inactivated form of the enzyme or noenzyme. Additional sensor system configurations that are possible with aplurality of working electrodes (that is, sensor elements) are describedin U.S. Patent Publication No. 2011-0024307 A1, which is incorporated byreference herein in its entirety.

FIGS. 9A-9C provide views of the in vivo portion of another embodimentof a multi-electrode sensor system 900 comprising two working electrodesand one reference electrode. As illustrated, the three electrodes areintegrated into one piece. The sensor system 900 comprises first,second, and third elongated bodies E1, E2, E3, each formed of aconductive core or of a core with a conductive layer deposited thereon.As illustrated in FIG. 9C, an insulating domain 910 and a membrane layer(not shown) can be deposited on top of an assembly comprising theelongated bodies E1, E2, E3. The insulating domain 910 can bind thethree elongated bodies E1, E2, E3 in close proximity of each other,while also separating them from direct contact with each other. Thematerials selected to form the insulating domain 910 may include any ofthe insulating materials described elsewhere herein, includingpolyurethane and polyimide, for example. Working electrode 904 onelongated body E1 and another working electrode (not shown) on elongatedbody E2, can be formed by removing portions of the insulating domain910, thereby exposing electroactive surface of the elongated bodies E1,E2. Similarly, the reference electrode 906 on elongated body E3 can alsobe formed by removing portions of the insulating domain 910, therebyexposing electroactive surface of the elongated body E3. By depositing amembrane layer comprising an enzyme on top of the assembly comprisingthe elongated bodies E1, E2, E3, the enzyme can cover at least a portionof the working and reference electrodes. Although not shown in FIG. 9Aand FIG. 9B, the distal ends 930′, 930″, 930′″ of the core portions ofthe elongated bodies E1, E2, E3, respectively, can be covered with aninsulating material (such as, for example, polyurethane or polyimide).Alternatively, the exposed core portions 930′, 930″, 930′″ can becovered with a membrane system and serve as additional working electrodesurface area.

As described elsewhere herein, the working electrodes, associated withthe elongated bodies can each have one or more characteristics thatdistinguish each working electrode from the other. For example, one ofthe working electrodes can have a membrane comprising an enzyme layerand the other working electrode can have a membrane comprising a layerhaving either an inactivated form of the enzyme or no enzyme. Additionalsensor system configurations that are possible with a plurality ofworking electrodes (that is, sensor elements) are described in U.S.Patent Publication No. 2011-0024307-A1, which is incorporated byreference herein in its entirety. In other embodiments, the workingelectrodes can be fabricated to have the same properties, therebyproviding a sensor system capable of providing redundancy of signalmeasurements.

FIGS. 10A-10C provide views of the in vivo portion of another embodimentof a multi-electrode sensor system 1000 comprising two workingelectrodes and at least one reference electrode. The sensor system 1000comprises first and second elongated bodies E1, E2, each formed of aconductive core or of a core with a conductive layer deposited thereon.An insulating layer 1010 can be deposited onto each elongated body E1,E2. Furthermore, a conductive domain 1020 and a membrane layer (notshown) can be deposited on top of an assembly comprising the elongatedbodies E1, E2 and the insulating layer 1010. The conductive domain 1020can bind the two elongated bodies E1, E2 into one elongated body. Theinsulating layers 1010 surrounding each elongated body E1, E2 canprevent electrical contact between the two elongated bodies E1, E2. Thematerials selected to form the insulating layer 1010 can include any ofthe insulating materials described elsewhere herein, includingpolyurethane and polyimide, for example. The materials selected to formthe conductive domain 1020 can include any of the conductive materialsdescribed elsewhere herein, including silver/silver chloride andplatinum, for example. Working electrode 1002′ on elongated body E1 andanother working electrode (not shown) on elongated body E2, can beformed by removing portions of the conductive domain 1020 and portionsof the insulating layer 810, thereby exposing electroactive surfaces ofelongated bodies E1, E2. The portion of the conductive domain 1020 notremoved forms the reference electrode. By depositing a membrane layercomprising an enzyme on top of the assembly comprising the elongatedbodies E1, E2, the enzyme can cover at least a portion of the workingand reference electrodes. With this particular sensor design, becausethe conductive domain 1020 is disposed between the contact point betweenthe two elongated bodies E1, E2, the sensor system's largestcross-sectional dimension is minimized, as compared to a design in whichboth of the elongated bodies were each individually covered with aconductive layer. Contacts 1004′, 1004″ are used to provide electricalconnection between the working electrodes and other components of thesensor system may be formed in a similar manner. As shown, contacts1004′ and 1004″ are separated from each other to prevent an electricalconnection therebetween. Although not shown in FIG. 10B, the distal ends1030′, 1030″ of the core portions of the elongated bodies E1, E2,respectively, can be covered with an insulating material (such as, forexample, polyurethane or polyimide). Alternatively, the exposed coreportions 1030′, 1030″ can be covered with a membrane system and serve asadditional working electrode surface area.

FIGS. 11A-11C provide views of the in vivo portion of another embodimentof a multi-electrode sensor system 1100 comprising two workingelectrodes and one reference electrode. The sensor system can comprise afirst, second, and third elongated bodies, each formed of a conductivecore or of a core with a conductive layer deposited thereon. Forexample, an insulating layer 1110 and a membrane layer (not shown) canbe deposited on top of the elongated bodies. The insulating layer 1110separates the elongated bodies from each other. The materials selectedto form the insulating layer 1110 can include any of the insulatingmaterials described elsewhere herein, including, for example,polyurethane and polyimide. Working electrodes 1102′, 1102″ andreference electrode 1106 can be formed by removing portions of theinsulating layer 1110, thereby exposing electroactive surface of theelongated bodies respectively. Contacts 1104′, 1104″ are used to provideelectrical connection between the working electrodes and othercomponents of the sensor system may be formed in a similar manner. Asshown, contacts 1104′ and 1104″ are separated from each other to preventan electrical connection therebetween. Although not shown in FIG. 11A,the distal ends 1130′, 1130″ of the core portions can be covered with aninsulating material (such as, for example, polyurethane or polyimide).Alternatively, the exposed core portions 1130′, 1130″ can be coveredwith a membrane system and serve as additional working electrode surfacearea.

To fabricate the sensor systems illustrated, e.g., in FIGS. 8A-8C,9A-9C, 10A-10C, and 11A-11C, the requisite number of elongated bodiescan be provided. As described above, the elongated bodies can be formedas an elongated conductive core, or alternatively as a core (conductiveor non-conductive) having at least one conductive material depositedthereon. The elongated bodies that correspond to working electrodes maycomprise an elongated core with a conductive material typically usedwith working electrodes (such as, for example, a core formed of aconductive material like platinum, or a core plated, coated, or claddedwith a conductive material like platinum). The elongated body thatcorresponds to a reference electrode may comprise an elongated coreplated, coated, or cladded with a silver/silver chloride conductivematerial.

Next, an insulating layer can be deposited onto each of the elongatedbodies. In some embodiments, the insulating or deposited layer can beformed of a thermoplastic material, thereby allowing the elongatedbodies to be attached together by a heating process that permits theinsulating layers of the elongated bodies to adhere together. In otherarrangements, the elongated bodies can be formed as an elongatedconductive core, or alternatively, as a core (conductive ornon-conductive) having at least one conductive material depositedthereon. Next, an insulating layer can be deposited onto each of theelongated bodies. Thereafter, a conductive layer can be deposited overthe insulating layer. In some embodiments, the elongated bodies can becoated with a thermoplastic material and fed through an aligning die.Afterwards, a conductive domain can be deposited over this singleelongated body. The coated domain is then allowed to dry or be cured,after which the one unitary elongated body is formed, in which the twoelongated bodies are encased and held together by conductive domain.

Thereafter, a layer removal process can be performed to remove portionsof the insulating or deposited layer. Any of the techniques describedherein (such as, for example, laser ablation, chemical etching, gritblasting) can be used. The insulating layer can be removed to form theworking electrode(s) and reference electrode. Contacts can be used toprovide electrical connection between the working electrodes and othercomponents of the sensor system may be formed in a similar manner.Contacts are separated from each other to prevent an electricalconnection therebetween. The layer removal process can be performed oneach individual elongated body, or a single geometrically complicatedelongated body.

After the conductive and insulating layers have been deposited onto theelongated body, and after selected portions of the deposited layers havebeen removed, a membrane or other layer(s) can be applied onto at leasta portion of the elongated bodies. In certain embodiments, the membraneor other layer(s) are applied only to the working electrodes, but inother embodiments the membrane or other layer(s) can be applied to theentire elongated body. In one embodiment, a membrane system is depositedonto the two working electrodes simultaneously while they are placedtogether (such as, by bundling). Alternatively, a membrane can bedeposited onto each individual working electrode, and the two workingelectrodes can then be placed together.

In one exemplary embodiment, two or more elongated bodies can be bundledtogether first (such as, for example, by providing adherence between theinsulating layers of the working electrodes) to form a subassembly. Anuncoated elongated conductive body can then be adhered to thesubassembly to form an assembly including all elongated bodies.Subsequently, a silver-comprising elongated conductive body can bechloridized to form a silver/silver chloride reference electrode.

In certain embodiments, the distal ends of the core portions of theelongated bodies can be covered with an insulating material (such as,for example, polyurethane or polyimide). In alternative embodiments, theexposed core portions can be covered with a membrane system or otherlayer and serve as, e.g., additional working or electrode surface area.

It should be understood that with any of the embodiments describedherein involving multiple working electrodes, one or more workingelectrodes may be designed to serve as an enzymatic electrode and one ormore working electrodes may be designed to serve as a “blank” workingelectrode configured to measure baseline. This configuration allows forsubtraction of a signal associated with the “blank” working electrode(that is, the baseline non-analyte related signal) from the signalassociated with the enzymatic-working electrode. The subtraction, inturn, results in a signal that contains substantially reduced (or no)non-analyte-related signal contribution (such as, for example,contribution from interferents).

Prevention of Cross Talk and Removal of Noise

In some circumstances, cross talk can interfere with analyte/noisedetection. In general, cross talk can occur when signal (for example, inthe form of a detectable species such as H₂O₂) is transferred from oneelectrode (for example, the enzyme coated silver/silver chloridereference electrode) to another (for example, the working electrode),and detected as a signal by the other electrode. To prevent cross talk,in certain embodiments, the silver/silver chloride reference electrodeand the working electrode can be spaced apart or can be separated by adiffusion barrier (such as an insulator, a non-conductive material, aresistance layer and/or the like). The diffusion barrier can be aphysical diffusion barrier, a spatial diffusion barrier, or a temporaldiffusion barrier, as discussed in more detail elsewhere herein.

In an electrochemical sensor system, noise can be recognized andsubstantially reduced and/or eliminated by a variety of sensorconfigurations and/or methods. Noise can be reduced and/or eliminated byusing, for example: 1) sensor configurations that can block and/orremove the interferent, or that can specifically detect the noise; and2) mathematical algorithms that can recognize and/or remove the signalnoise component. Devices and methods for reducing and/or eliminatingnoise can be provided, such as blocking interferent passage to thesensor's electroactive surfaces, diluting and/or removing interferentsaround the sensor, and mathematically determining and eliminating thenoise signal component. Various sensor structures (for example, multipleworking electrodes, membrane interference domains, etc.), bioactiveagents, algorithms and the like, disclosed elsewhere herein, can beemployed in a plurality of combinations, depending upon the desiredeffect and the noise reduction strategy selected.

In one embodiment, the sensor system comprises a silver/silver chloridereference electrode covered at least in part with an enzyme layer, aworking electrode with enzyme over its electroactive surface, and aninterference domain configured to substantially block interferentpassage therethrough, such that at least some interferent no longer hasa substantial effect on sensor measurements (for example, at the workingelectrode). The interference domain can be a component of the membranesystem, such as illustrated in FIGS. 6A-6C, and can be disposed at anylevel (that is, layer or domain) of the membrane system (for example,more proximal or more distal to the electroactive surfaces than asillustrated in FIG. 6A-6C). In some embodiments, the interference domainis combined with an additional membrane domain, such as the resistancedomain or the enzyme domain.

In another aspect, the sensor can be configured to reduce noise,including non-constant non-analyte related noise with an overlappingmeasuring potential with the analyte. A variety of noise can occur whena sensor has been implanted in a host. Generally, implantable sensorsmeasure a signal (that is, counts) that generally comprises at least twocomponents, the background signal (that is, background noise) and theanalyte signal. The background signal can be composed substantially ofsignal contribution due to factors other than glucose (for example,non-reaction-related hydrogen peroxide generated in connection withregeneration of the reference electrode). The analyte signal (forexample, glucose) can be composed substantially of signal contributiondue to the analyte. Consequently, because the signal includes these twocomponents, a calibration can be performed in order to determine theanalyte (for example, glucose) concentration by solving for the equationy=mx+b, where the value of b represents the background of the signal.

In some circumstances, the background is comprised of both constant (forexample, baseline) and non-constant (for example, noise) factors.Generally, it is desirable to remove the background signal to provide amore accurate analyte concentration to the host and/or health careprofessional.

Baseline does not significantly adversely affect the accuracy of thecalibration of the analyte concentration because baseline can berelatively constantly eliminated using the equation y=mx+b. In contrast,noise can be difficult to remove from the sensor signal by calibrationusing standard calibration equations, for example, because thebackground of the signal does not remain constant. Noise cansignificantly adversely affect the accuracy of the calibration of theanalyte signal. Additionally noise can occur in the signal ofconventional sensors with electrode configurations that are notparticularly designed to measure noise substantially equally at bothactive and in-active electrodes (for example, wherein the electrodes arespaced and/or non-symmetrical, noise may not be equally measured andtherefore not easily removed using conventional dual-electrode designs).

There are a variety of ways noise can be recognized and/or analyzed. Forexample, the sensor data stream can be monitored, signal artifacts canbe detected, and data processing can be based at least in part onwhether or not a signal artifact has been detected, such as described inU.S. Pat. No. 8,101,174 and U.S. Patent Publication No. 2007-0027370-A1,herein incorporated by reference in their entirety.

Accordingly, if a sensor is designed such that the signal contributiondue to baseline and noise can be removed, then more accurate analyteconcentration data can be provided to the host and/or a healthcareprofessional.

In some embodiments, an analyte sensor (for example, glucose sensor) isconfigured for insertion into a host for measuring an analyte (forexample, glucose) in the host. The sensor includes a silver/silverchloride reference electrode disposed at least partially beneath anactive enzymatic portion of a membrane on the sensor, a first workingelectrode disposed beneath an active enzymatic portion of a membrane onthe sensor, a second working electrode disposed beneath an inactive- ornon-enzymatic portion of the membrane on the sensor, and electronicsoperably connected to the first and second working electrode andconfigured to process the first and second signals to generate ananalyte (for example, glucose) concentration substantially withoutsignal contribution due to non-glucose related noise artifacts. Noisedue to biochemical/chemical factors, e.g., hydrogen peroxide generatedby the enzyme-containing membrane covering the reference electrode, canimpinge upon the two working electrodes of certain embodiments (forexample, on with and one without active enzyme) to about the same extentbecause of the sensor's small size and symmetrical configuration.Accordingly, the sensor electronics can use these data to calculate theglucose-only signal, as described elsewhere herein. The signalcorresponding to the hydrogen peroxide generated by theenzyme-containing membrane covering the reference electrode can then becanceled out by use of the signal from the second working electrode.

In one exemplary embodiment, the analyte sensor is a glucose sensor thatcan measure a first signal associated with both glucose and non-glucoserelated electroactive compounds having a first oxidation/reductionpotential. The glucose sensor can also measure a second signal, whichcan be associated with background noise of the glucose sensor. Thebackground noise can be composed of signal contribution due tonon-reaction-related hydrogen peroxide. The first and second workingelectrodes can integrally form at least a portion of the sensor, such asbut not limited to the in vivo portion of the sensor, as discussedelsewhere herein, and employed in combination with a silver/silverchloride reference electrode covered at least in part with an enzymelayer.

Furthermore, the sensor can have a diffusion barrier that cansubstantially block (for example, attenuate) diffusion of glucose orH₂O₂ between the first and second working electrodes. Advantageously,the sensor can also have a diffusion barrier that can substantiallyblock (for example, attenuate) diffusion of glucose or H₂O₂ between thesilver/silver chloride reference electrode and the first workingelectrode. In various embodiments, the sensor includes a diffusionbarrier configured to be physical, spatial, and/or temporal.

FIG. 16 schematically illustrates one embodiment of a sensor (forexample, a portion of an in vivo portion of a sensor) having one or morecomponents that can act as a diffusion barrier (for example, preventdiffusion of electroactive species from one electrode to another). Asilver/silver chloride reference electrode E1 can be covered at least inpart by a membrane layer 10000 comprising active enzyme. For example, ina glucose sensor, the silver/silver chloride reference electrode E1 canbe coated with glucose oxidase enzyme (GOx). Although not illustrated, aworking electrode E2 can also be disposed beneath a membrane layer10000. The working electrode E2 can be separated from the silver/silverchloride reference electrode E1 by a diffusion barrier D, such as, butnot limited, to a physical diffusion barrier (for example, a layer ofnon-conductive material/insulator). The diffusion barrier D can also bespatial or temporal, as discussed elsewhere herein.

Glucose and oxygen can diffuse into the membrane layer 10000, where theycan react with GOx to produce gluconate and H₂O₂. At least a portion ofthe H₂O₂ can diffuse to the silver/silver chloride reference electrodeE1 where it can reduce silver ion to silver metal. Advantageously,silver metal can react with chloride to regenerate silver chloride atthe surface of the silver/silver chloride reference electrode E1. Theremaining H₂O₂ can diffuse to other locations in the membrane layer10000 or out of the membrane layer 10000 (illustrated by the wavyarrows). Without a diffusion barrier D, a portion of the H₂O₂ candiffuse to the working electrode E2, where it can be electrochemicallyoxidized to oxygen and transfer two electrons (2e⁻) to the workingelectrode E2, which can result in a glucose signal that is recorded bythe sensor electronics (not shown). As a result, an aberrant signal canbe recorded by the sensor electronics that can be interpreted as anartificially high glucose concentration (for example, cross talk ornoise).

A substantial diffusion barrier D between the silver/silver chloridereference electrode E1 and the working electrode E2 can be provided,such that the H₂O₂ cannot substantially diffuse from the silver/silverchloride reference electrode E1 to the working electrode E2.Accordingly, the possibility of an aberrant signal produced by H₂O₂ fromthe silver/silver chloride reference electrode E1 at the workingelectrode E2 is reduced or avoided.

A variety of diffusion barriers can be employed to prevent cross talk ornoise. For example, the diffusion barrier D can be a physical diffusionbarrier, such as a structure between the silver/silver chloridereference electrode E1 and the working electrode E2, that can blockglucose and H₂O₂ from diffusing from the silver/silver chloridereference electrode E1 to the working electrode E2. In some embodiments,a physical diffusion barrier is formed of one or more membranematerials, such as those used in formation of an interference domainand/or a resistance domain. Such materials can include, but are notlimited to, silicones, polyurethanes, cellulose derivatives (cellulosebutyrates and cellulose acetates, and the like) and combinationsthereof, as described elsewhere herein. In some embodiments, thephysical diffusion barrier includes one or more membrane domains. Forexample, the physical diffusion barrier can be a discontinuous portionof the membrane (for example, separate, distinct or discontinuousmembrane structures) disposed between the silver/silver chloridereference electrode and the working electrode, and can include one ormore membrane portion(s) (for example, interference and/or resistancedomains). In some embodiments, the physical diffusion barrier includesfirst and second barrier layers formed independently on thesilver/silver chloride reference electrode and the working electrode. Insome embodiments the barrier layer is the resistance domain. In stillother embodiments, the physical diffusion barrier can be a continuousmembrane (and/or membrane domain(s)) disposed between the silver/silverchloride reference electrode and the working electrode. In someembodiments, the physical diffusion barrier attenuates (for example,suppresses, blocks, prevents) diffusion of the H₂O₂ (for example, crosstalk) by at least 2-fold, or at least 5-fold, or even at least 10-fold.In some embodiments, the physical diffusion barrier attenuates crosstalkat least about 50%, at least about 75%, at least about 80%, at leastabout 90%, at least about 95%, or at least about 99%.

Alternatively, the diffusion barrier D can be a spatial diffusionbarrier, such as a distance between the silver/silver chloride referenceelectrode E1 and the working electrode E2 that can block glucose andH₂O₂ from diffusing from the first silver/silver chloride referenceelectrode E1 to the working electrode E2. For example, a spatialdiffusion barrier can be created by separating the silver/silverchloride reference electrode and the working electrode by a distancethat is too great for the H₂O₂ to substantially diffuse therebetween. Insome embodiments, the spatial diffusion barrier is about 0.01, about0.02, about 0.03, about 0.04, about 0.05, about 0.06, about 0.07, orabout 0.08 inches to about 0.09, about 0.10, about 0.11, or about 0.120inches. In other embodiments, the spatial diffusion barrier is about0.020 inches to about 0.050 inches. Still in other embodiments, thespatial diffusion barrier is about 0.055 inches to about 0.095 inches. Anon-conductive material (for example, a polymer structure or coatingsuch as Parylene) can be configured to act as a spatial diffusionbarrier.

In other embodiments, the diffusion barrier D can be a temporaldiffusion barrier, such as a period of time between the activity of thesilver/silver chloride reference electrode E1 and the working electrodeE2 such that if glucose or H₂O₂ were to diffuse from the silver/silverchloride reference electrode E1 to the working electrode E2, the workingelectrode E2 would not substantially be influenced by the H₂O₂ from thesilver/silver chloride reference electrode E1.

In some embodiments, the dual-electrode sensor can comprise aninsulator, such as an electrical insulator, located between thesilver/silver chloride reference electrode and the working electrode,wherein the insulator can comprise a physical diffusion barrier. Thephysical diffusion barrier can be configured to structurally block asubstantial amount of diffusion of at least one of an analyte (forexample, glucose) and a co-analyte (for example, H₂O₂) between thesilver/silver chloride reference electrode and the working electrode.The diffusion barrier can comprise a structure that protrudes from aplane that intersects both the silver/silver chloride referenceelectrode and the working electrode. Moreover, the structure thatprotrudes can comprise an electrical insulator and/or an electrode.

In some embodiments, the sensor can comprise an insulator locatedbetween the silver/silver chloride reference electrode and the workingelectrode. The insulator can comprise a diffusion barrier configured tosubstantially block diffusion of at least one of an analyte (forexample, glucose) and a co-analyte (for example, H₂O₂) between thesilver/silver chloride reference electrode and the working electrode.The diffusion barrier can be a temporal diffusion barrier configured toblock or avoid a substantial amount of diffusion or reaction of at leastone of the analyte (for example, glucose) and the co-analyte (forexample, H₂O₂) between the silver/silver chloride reference electrodeand the working electrode.

In other embodiments, the electrochemical sensor can comprise a sensormembrane configured to substantially block diffusion of at least one ofan analyte (for example, glucose) and a co-analyte (for example, H₂O₂)between the silver/silver chloride reference electrode and the workingelectrode by a discontinuity of the sensor membrane between thesilver/silver chloride reference electrode and the working electrode. Adiscontinuity of the sensor membrane can be a type of physical diffusionbarrier formed by a portion of the membrane between the silver/silverchloride reference electrode and the working electrode, for example,wherein a discontinuity in the membrane structure blocks diffusion ofH₂O₂ between the silver/silver chloride reference electrode and theworking electrode.

In some embodiments, the sensor is an indwelling sensor, such asconfigured for insertion into the host's circulatory system via a veinor an artery. In some exemplary embodiments, an indwelling sensorincludes at least two working electrodes that are inserted into thehost's blood stream through a catheter. The sensor includes at least asilver/silver chloride reference electrode that can be disposed eitherwith the working electrodes or remotely from the working electrodes. Thesensor includes a spatial, a physical, or a temporal diffusion barrier.A spatial diffusion barrier can be configured as described in U.S.Patent Publication No. 2008-0083617-A1, which is incorporated byreference herein in its entirety.

To configure a spatial diffusion barrier between the silver/silverchloride reference electrode and the working electrode, the location ofthe silver/silver chloride reference electrode and the working electrodecan be dependent upon the orientation of the sensor after insertion intothe host's artery or vein. For example, in an embodiment configured forinsertion in the host's blood flow (for example, in an artery or vein),the silver/silver chloride reference electrode can be downstream fromthe working electrode (for example, relative to the direction of bloodflow). Due to this configuration, H₂O₂ produced the silver/silverchloride reference electrode would be carrier downstream (e.g., awayfrom the working electrode) and thus not affect the working electrode.

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Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. applicationSer. No. 09/447,227 filed on Nov. 22, 1999 and entitled “DEVICE ANDMETHOD FOR DETERMINING ANALYTE LEVELS”; U.S. application Ser. No.12/828,967 filed on Jul. 1, 2010 and entitled “HOUSING FOR ANINTRAVASCULAR SENSOR”; U.S. application Ser. No. 13/461,625 filed on May1, 2012 and entitled “DUAL ELECTRODE SYSTEM FOR A CONTINUOUS ANALYTESENSOR”; U.S. application Ser. No. 13/594,602 filed on Aug. 24, 2012 andentitled “POLYMER MEMBRANES FOR CONTINUOUS ANALYTE SENSORS”; U.S.application Ser. No. 13/594,734 filed on Aug. 24, 2012 and entitled“POLYMER MEMBRANES FOR CONTINUOUS ANALYTE SENSORS”; U.S. applicationSer. No. 13/607,162 filed on Sep. 7, 2012 and entitled “SYSTEM ANDMETHODS FOR PROCESSING ANALYTE SENSOR DATA FOR SENSOR CALIBRATION”; U.S.application Ser. No. 13/624,727 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/624,808 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/624,812 filed on Sep. 21, 2012 and entitled“SYSTEMS AND METHODS FOR PROCESSING AND TRANSMITTING SENSOR DATA”; U.S.application Ser. No. 13/732,848 filed on Jan. 2, 2013 and entitled“ANALYTE SENSORS HAVING A SIGNAL-TO-NOISE RATIO SUBSTANTIALLY UNAFFECTEDBY NON-CONSTANT NOISE”; U.S. application Ser. No. 13/733,742 filed onJan. 3, 2013 and entitled “END OF LIFE DETECTION FOR ANALYTE SENSORS”;U.S. application Ser. No. 13/733,810 filed on Jan. 3, 2013 and entitled“OUTLIER DETECTION FOR ANALYTE SENSORS”; U.S. application Ser. No.13/742,178 filed on Jan. 15, 2013 and entitled “SYSTEMS AND METHODS FORPROCESSING SENSOR DATA”; U.S. application Ser. No. 13/742,694 filed onJan. 16, 2013 and entitled “SYSTEMS AND METHODS FOR PROVIDING SENSITIVEAND SPECIFIC ALARMS”; U.S. application Ser. No. 13/742,841 filed on Jan.16, 2013 and entitled “SYSTEMS AND METHODS FOR DYNAMICALLY ANDINTELLIGENTLY MONITORING A HOST'S GLYCEMIC CONDITION AFTER AN ALERT ISTRIGGERED”; and U.S. application Ser. No. 13/747,746 filed on Jan. 23,2013 and entitled “DEVICES, SYSTEMS, AND METHODS TO COMPENSATE FOREFFECTS OF TEMPERATURE ON IMPLANTABLE SENSORS”.

The above description presents the best mode contemplated for carryingout the present invention, and of the manner and process of making andusing it, in such full, clear, concise, and exact terms as to enable anyperson skilled in the art to which it pertains to make and use thisinvention. This invention is, however, susceptible to modifications andalternate constructions from that discussed above that are fullyequivalent. Consequently, this invention is not limited to theparticular embodiments disclosed. On the contrary, this invention coversall modifications and alternate constructions coming within the spiritand scope of the invention as generally expressed by the followingclaims, which particularly point out and distinctly claim the subjectmatter of the invention. While the disclosure has been illustrated anddescribed in detail in the drawings and foregoing description, suchillustration and description are to be considered illustrative orexemplary and not restrictive.

All references cited herein are incorporated herein by reference intheir entirety. To the extent publications and patents or patentapplications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

Unless otherwise defined, all terms (including technical and scientificterms) are to be given their ordinary and customary meaning to a personof ordinary skill in the art, and are not to be limited to a special orcustomized meaning unless expressly so defined herein. It should benoted that the use of particular terminology when describing certainfeatures or aspects of the disclosure should not be taken to imply thatthe terminology is being re-defined herein to be restricted to includeany specific characteristics of the features or aspects of thedisclosure with which that terminology is associated. Terms and phrasesused in this application, and variations thereof, especially in theappended claims, unless otherwise expressly stated, should be construedas open ended as opposed to limiting. As examples of the foregoing, theterm ‘including’ should be read to mean ‘including, without limitation,’‘including but not limited to,’ or the like; the term ‘comprising’ asused herein is synonymous with ‘including,’ ‘containing,’ or‘characterized by,’ and is inclusive or open-ended and does not excludeadditional, unrecited elements or method steps; the term ‘having’ shouldbe interpreted as ‘having at least;’ the term ‘includes’ should beinterpreted as ‘includes but is not limited to;’ the term ‘example’ isused to provide exemplary instances of the item in discussion, not anexhaustive or limiting list thereof; adjectives such as ‘known’,‘normal’, ‘standard’, and terms of similar meaning should not beconstrued as limiting the item described to a given time period or to anitem available as of a given time, but instead should be read toencompass known, normal, or standard technologies that may be availableor known now or at any time in the future; and use of terms like‘preferably,’ ‘preferred,’ ‘desired,’ or ‘desirable,’ and words ofsimilar meaning should not be understood as implying that certainfeatures are critical, essential, or even important to the structure orfunction of the invention, but instead as merely intended to highlightalternative or additional features that may or may not be utilized in aparticular embodiment of the invention. Likewise, a group of itemslinked with the conjunction ‘and’ should not be read as requiring thateach and every one of those items be present in the grouping, but rathershould be read as ‘and/or’ unless expressly stated otherwise. Similarly,a group of items linked with the conjunction ‘or’ should not be read asrequiring mutual exclusivity among that group, but rather should be readas ‘and/or’ unless expressly stated otherwise.

Where a range of values is provided, it is understood that the upper andlower limit, and each intervening value between the upper and lowerlimit of the range is encompassed within the embodiments.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity. The indefinite article ‘a’ or ‘an’ does not exclude aplurality. A single processor or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage. Anyreference signs in the claims should not be construed as limiting thescope.

It will be further understood by those within the art that if a specificnumber of an introduced claim recitation is intended, such an intentwill be explicitly recited in the claim, and in the absence of suchrecitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases ‘at least one’ and ‘one or more’ to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles ‘a’ or ‘an’ limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases‘one or more’ or ‘at least one’ and indefinite articles such as ‘a’ or‘an’ (e.g., ‘a’ and/or ‘an’ should typically be interpreted to mean ‘atleast one’ or ‘one or more’); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of ‘two recitations,’ without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to ‘at least one of A, B, and C, etc.’ is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., ‘a system having at least one ofA, B, and C’ would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to ‘at least one of A, B, or C, etc.’ is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., ‘a system having at leastone of A, B, or C’ would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase ‘A or B’ will be understood toinclude the possibilities of ‘A’ or ‘B’ or ‘A and B.’

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term ‘about.’ Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

Furthermore, although the foregoing has been described in some detail byway of illustrations and examples for purposes of clarity andunderstanding, it is apparent to those skilled in the art that certainchanges and modifications may be practiced. Therefore, the descriptionand examples should not be construed as limiting the scope of theinvention to the specific embodiments and examples described herein, butrather to also cover all modification and alternatives coming with thetrue scope and spirit of the invention.

What is claimed is:
 1. An electrochemical sensor for measuring ananalyte concentration in a host, comprising: a working electrodeconfigured to measure a concentration of an analyte; and a referenceelectrode comprising a material that depletes during sensor use, whereinthe material is configured to be regenerated during sensor use, andwherein a rate of material depletion during sensor use substantiallycorrelates with a rate of material regeneration during sensor use over atime period.
 2. The electrochemical sensor of claim 1, wherein acorrelation between the rate of material regeneration and the rate ofmaterial depletion is positive.
 3. The electrochemical sensor of claim1, wherein the sensor is configured to increase the rate of materialregeneration responsive to an increase in the rate of materialdepletion.
 4. The electrochemical sensor of claim 1, wherein the sensoris configured to decrease the rate of material regeneration responsiveto a decrease in the rate of material depletion.
 5. The electrochemicalsensor of claim 1, wherein at least a portion of the reference electrodeis covered with an enzyme layer.
 6. The electrochemical sensor of claim5, wherein the enzyme is an oxidase enzyme.
 7. The electrochemicalsensor of claim 1, wherein the analyte is glucose.
 8. Theelectrochemical sensor of claim 1, wherein the material is silverchloride.
 9. The electrochemical sensor of claim 1, wherein thereference electrode is formed of a chloridized elongated silver body.10. The electrochemical sensor of claim 1, wherein the enzyme layer hasa thickness from about 0.01 microns to about 12 microns thick.
 11. Theelectrochemical sensor of claim 1, wherein a percentage of a surfacearea of the reference electrode covered with the enzyme layer is fromabout 10% to about 100%.